Add instructions for removing pyc files to docs

So an issue I hit when following the getting started docs for developers
is that I forgot to remove old pyc files and that can cause
keystone-manage db_sync to throw errors.  I added some extra doc that
explains how to remove these files should this situation be encountered

Change-Id: Ib41c62e99faa7c82f89ab3c5bfa0d740f426829a
Merge "Add instructions for removing pyc files to docs"