add HACKING document to the zeitgeist project

To define some principles for existing and hopefully at some point new contributors it would be cool to have a HACKING document.
Let's use this bugreport to collect a few things which should be mentioned there:

 * python version: zeitgeist is developed to be usable with python >= 2.5
 * code review: bigger changes should be reviewed by at least one member of the launchpad.net/~zeitgeist team
 * changes to the public API are only possible major release series
 * doc- or unitests should be written for significant changes to the code, the testsuite should be run before commits to make sure everything works fine
 * code should be documented as much as possible

does this make sense?
any thoughts or additional things we should mention?