Added interfaces/README with commit-deletion notes.

Added interfaces/README with commit-deletion notes.

Up to now, my email interface never committed automatically, in order
to avoid locking in inappropriate changes.  However, with the ability
to modify bug status, etc., it could be hard to determine the correct
status with a single email's effects removed.  In order to make that
easier, I'm switching over to a "auto-commit after every user action"
model, and I've looked up the incantations for commit deletion for bzr
and git (the VCSs I use).  These incantations are recorded in
interfaces/README.

Next up: add auto-commit functionality.