Added .gitignore for docs generated in _build.
Added .gitignore for docs generated in _build.

dwhswenson/openpathsampling,choderalab/openpathsampling,jhprinz/openpathsampling,openpathsampling/openpathsampling,choderalab/openpathsampling,jhprinz/openpathsampling,jhprinz/openpathsampling,dwhswenson/openpathsampling,openpathsampling/openpathsampling,openpathsampling/openpathsampling,choderalab/openpathsampling,openpathsampling/openpathsampling,dwhswenson/openpathsampling,dwhswenson/openpathsampling