Document workarounds for installing Gephi on Linux

[wetneb]

Originally posted by @cschwem2er at gephi/gephi#1417:

As several users report crashes when using Linux (https://github.com/gephi/gephi/issues1192) and a temporary fix was suggested by @3ruce, it would be very nice to have a short tutorial for two things:

1. How to add Gephi as a PPA repository, so that linux (ubuntu/ mint) users always get the latest version
2. How to enable options like "LIBGL_ALWAYS_SOFTWARE=1" without having to start Gephi via the terminal each time.