Add CODEOWNERS file

To make changelog preparation less stressful let's pass each and every
change to changelogs/ directly through the doc team.

NO_CHANGELOG=no code changes
NO_TEST=no code changes
NO_DOC=no code changes

.github/CODEOWNERS

+# Documentation team is in charge of changelogs.
+# No changes should go w/o approval by doc team
+/changelogs/ @tarantool/doc