Prepare explain command section of documentation.

Though SQL command list contains EXPLAIN, there is no good description how to read explain output.

1. Need to explain the user how to read the plans. I.e. we want to form a guide for the user how to read each line in the plan from the simplest one (select * from t) to some complicated examples with joins, subqueries and aggregates. These examples would be introduced step-by-step in the next issues.
2. Mention that explain doesn't really execute the plan, but rather builds its plan on the router node.
3. Introduce relational operators that modify pulled up tuples (SCAN, SELECTION, PROJECTION, etc) and expressions (i.e. tuple modifiers).