Add .gitignore for unofficial mirrors.
|Reported by:||Adam Vandenberg||Owned by:||nobody|
|Cc:||Triage Stage:||Design decision needed|
|Has patch:||yes||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
I mostly expect this to be "won't fixed", but it would be very useful for unofficial git mirrors if Django had at least a basic .gitignore file that excluded .py[c|o] files.