PARQUET-113: Add specs for LIST and MAP annotations.

Draft specs for using `MAP` and `LIST` annotations. This doesn't include a `LIST_ELEMENT` addition to the thrift definition, it is documentation only.

Please help verify that this can read all existing map and list data correctly!
This conflicts with the above where it says "The outer-most level *must* be a group annotated with `LIST` that contains a single field named `array`" (my emphasis).