Add note about developing on a production machine

This is the bit of documentation missing for issue 170. Doing
development on a machine which already has Ganeti installed kind of
works, but only when the installed and the developed version are very
similar, and even then it can be problematic.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>