[rfc] better getting-started-with-development guide

[Martin Pool]

https://code.edge.launchpad.net/~mbp/bzr/doc/+merge/19682 updates some
existing "how to modify bazaar" documentation.

I would enormously appreciate it if people who have recently sent
their first contribution to Bazaar could read it and tell me either if
it would have helped them, or if there was something they found hard,
or still find hard, that should be described too.

-- 
Martin <http://launchpad.net/~mbp/>