add enums
add delta encoding rationale
add BIT_PACKED and RLE
move level encodings to encoding.md
move encodings
move encodings
add encodings link
fix language
Merge pull request #79 from Parquet/encodings_reformulate

Reorganize encodings and add details
[maven-release-plugin] prepare release parquet-format-2.0.0
[maven-release-plugin] prepare for next development iteration