package: add git hook instructions

[srl295]

Add instructions and a hook to call core-validate-commit from
git commits.

Depends on #11

Known issues:

• leaves ${TMPF} around (temporary file)
• probably will reject fixup: type commit logs
• may be too strict if someone is just doing a simple
  'work in progress' kind of local branch

Larger improvement:

• fold the entire hook script into a mode, and/or an alternate
  script entrypoint into core-validate-commit. This would drop
  requirements on sh, sed, grep…

Fixes: #12