doc: add contributing guide

GitHub suggests to look on the document, when it is present. The banner
is shown for visitors of issues and pull requests pages. We can use this
feature to highlight typical stones on the contributor's road.

My goal is to leave suggestions for typical points of a confusion in a
short document (don't bury into details) and leave links for further
reading.

I hope we should move most of content of the 'open a pull request'
section to a wiki page and possibly merge it with [1] and [2]. It
requires more work and I would consider it as a future step toward good
developer guidelines.

[1]: https://github.com/tarantool/tarantool/wiki/Code-review-procedure
[2]: https://www.tarantool.io/en/doc/latest/dev_guide/developer_guidelines

NO_CHANGELOG=not a user visible change
NO_DOC=nothing requires an update on the website
NO_TEST=it is purely documentation change

CONTRIBUTING.md

+# Contributing to Tarantool
+
+You likely come here from the issues or pull requests page. There are some
+suggestions on how to make your contribution most useful.
+
+## File an issue
+
+Consider using a bug report/feature request template. Feel free to adapt it for
+your needs. You can opt out and start from a blank issue, but be mindful of the
+completeness of the information.
+
+There are feature requests of different kinds:
+
+* A complement to existing functionality or another ready-to-implement request.
+* A new idea or something else that requires a discussion.
+
+The former is completely okay to be asked via an issue. For the latter kind of
+a request consider opening a discussion in the [Ideas][ideas] category. It
+helps to keep the issue tracker actual and clean rather than a bag of thoughts.
+
+Questions could be asked in discussions in the [Q&A][q-a] category.
+
+## Open a pull request
+
+There are many aspects which a programmer should take care of. Here is a short
+list of simple rules, which aims to reduce ping-pong between a developer and a
+reviewer.
+
+1. Target a pull request to the `master` branch.
+2. Don't forget to:
+   - Add a [changelog entry][changelogs]. Follow suggestions from the
+     [How to write release notes][release-notes] document.
+   - Add a [documentation request][docbot] into the commit message. Example:
+
+     ```
+     @TarantoolBot document
+     Title: One-line title
+     Description that can be multiline,
+     contain markup and links.
+     ```
+   - Add a test (as for a feature, as well as for a bugfix). Prefer `luatest`
+     over other test types.
+
+   Add `NO_CHANGELOG=<reason>`, `NO_DOC=<reason>`, and/or `NO_TEST=<reason>`
+   into a commit message if some of those bullets are not applicable. Every
+   commit in a patchset is validated against those rules.
+3. Adhere [How to write a commit message][commit-message] rules. Highlights:
+   - Describe a reason of the change, **why** it is necessary (see rule 8). The
+     real reason. Bad example: 'it reaches end-of-life'. Good example: 'it
+     reaches end-of-life and its support is problematic due to the following
+     problems: X, Y, Z'.
+   - Follow 50/72 rule (see rules 2 and 7). 50 chars length for the title is
+     not a strict requirement, but a good soft limit. 72 chars for the commit
+     body is the strict requirement for a prose text, but it can be overrun for
+     listings such as code or logs.
+4. Look at the code around and mimic the style. If in doubt, consult with the
+   style guides for [C][c-style] and [Lua][lua-style].
+5. Don't hesitate to ask for help. If you have doubts, highlight them for a
+   reviewer in the commit message, the pull request description or pull request
+   comments.
+6. Wait for CI and fix all problems.
+   - Glance on the [checkpatch][checkpatch] output if the CI job is red.
+   - If there are test fails that look irrelevant to the changes, highlight
+     this fact in the pull request comments.
+
+Once you received a review, follow the next suggestions to make the process
+comfortable for everyone:
+
+1. Don't add fixup commits on top of the initial patchset. Squash fixups into
+   appropriate commits and force-push your branch. Your patchset will land to
+   `master` as is, without any squashing or reformatting. Keep it in a good
+   shape.
+2. React to comments and respond with a summary of changes. If you disagree with
+   a comment, describe your arguments or doubts.
+3. Work in iterations. Either process all comments at once or mark a pull
+   request as a draft and return it back when all comments will be answered. A
+   reviewer always wants to just look and say 'everything is nice' rather than
+   request changes and remind about forgotten things.
+4. If you reached out of spare time, mark the pull request as draft or close
+   it.
+
+## Useful links
+
+* [Good first issues][good-first-issues] -- if you search for a task to learn
+  Tarantool's code.
+* [How to self-review][self-review] -- more detailed recommendations how to
+  make nice patches.
+* [How to get involved][get-involved] -- what else goes on around Tarantool and
+  how to get involved.
+
+[ideas]: https://github.com/tarantool/tarantool/discussions/categories/ideas
+[q-a]: https://github.com/tarantool/tarantool/discussions/categories/q-a
+[changelogs]: doc/changelogs.md
+[release-notes]: https://www.tarantool.io/en/doc/latest/contributing/release_notes/
+[docbot]: https://github.com/tarantool/docbot
+[commit-message]: https://www.tarantool.io/en/doc/latest/dev_guide/developer_guidelines/#how-to-write-a-commit-message
+[c-style]: https://www.tarantool.io/en/doc/latest/dev_guide/c_style_guide/
+[lua-style]: https://www.tarantool.io/en/doc/latest/dev_guide/lua_style_guide/
+[checkpatch]: https://github.com/tarantool/checkpatch
+[good-first-issues]: https://github.com/tarantool/tarantool/wiki/Good-first-issues
+[self-review]: https://github.com/tarantool/tarantool/wiki/Code-review-procedure
+[get-involved]: https://www.tarantool.io/en/doc/latest/contributing/contributing/

README.md

@@ -57,8 +57,8 @@ Please report bugs to our [issue tracker][issue-tracker]. We also warmly
 welcome your feedback on the [discussions][discussions-url] page and questions
 on [Stack Overflow][stackoverflow-url].
 
-We accept contributions via pull requests. Check out our [How to get
-involved][get-involved] guide.
+We accept contributions via pull requests. Check out our [contributing
+guide][contributing].
 
 Thank you for your interest in Tarantool!
 
@@ -84,5 +84,5 @@ Thank you for your interest in Tarantool!
 [download]: https://www.tarantool.io/en/download/
 [building]: https://www.tarantool.io/en/doc/latest/dev_guide/building_from_source/
 [issue-tracker]: https://github.com/tarantool/tarantool/issues
-[get-involved]: https://www.tarantool.io/en/doc/latest/contributing/contributing/
+[contributing]: CONTRIBUTING.md
 [awesome-list]: https://github.com/tarantool/awesome-tarantool/