Move command reference to COMMANDS.md
README.md: Minor formatting
README.md: Add dividers between major sections
COMMANDS.md: Minor formatting
COMMANDS.md: Add commands from `pyenv --help`
COMMANDS.md: Match order of commands from `pyenv --help`

Also added `help` output for `pyenv {,un}install`, since those sections didn’t exist before.

They should probably be revised at some point.  In the meantime, I think something is better than nothing (in this case).
Merge pull request #141 from Zearin/docs

Updated README.md; Commands moved to COMMANDS.md.