Update dovecot docs with a note about configuring postfix
have install-into-gaia trigger the clean target

It's very possible to get a stale dependency in install-into-gaia. While it
does make us a smidge slower, we should do this until we can have ironclad
dependencies so we never screw up.
Merge pull request #212 from gaye/postfix-config-readme

Update dovecot docs with a note about configuring postfix r=asuth
Merge pull request #218 from asutherland/install-clean

have install-into-gaia trigger the clean target
remove moot comments, move submodules docs up
contributing info
Merge pull request #220 from asutherland/hackability-docs

Hackability docs.  This is either stuff that was agreed-upon or I think documents the state of things or recommended patch submission process.  Issue pull requests for things that are wrong/debatable.