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>

doc/devnotes.rst

@@ -66,6 +66,13 @@ different python version)::
   $ ./autogen.sh && \
     ./configure --prefix=/usr/local --sysconfdir=/etc --localstatedir=/var
 
+Note that doing development on a machine which already has Ganeti
+installed is problematic, as ``PYTHONPATH`` behaviour can be confusing
+(see Issue 170 for a bit of history/details; in general it works if
+the installed and developed versions are very similar, and/or if
+PYTHONPATH is customised correctly). As such, in general it's
+recommended to use a "clean" machine for ganeti development.
+
 Haskell development notes
 -------------------------