Add a note about git commit message style.
Add a note about git commit message style.

Update contributing.rst to point to Tim Pope's suggestions
for how to write git commit messages.

Change-Id: Idcbece3e7576f54794d8a107aa2c5aa48dc4ddf7

timfel/ceres-bin,entylop/ceres-solver,stonier/ceres_solver,timfel/ceres-bin,DougFirErickson/ceres-solver,whuaegeanse/ceres-solver,ysonggit/ceres-solver,DougFirErickson/ceres-solver,stonier/ceres_solver,timfel/ceres-bin,ysonggit/ceres-solver,ysonggit/ceres-solver,DougFirErickson/ceres-solver,timfel/ceres-bin,whuaegeanse/ceres-solver,entylop/ceres-solver,stonier/ceres_solver,kartikkumar/ceres-solver,kartikkumar/ceres-solver,ysonggit/ceres-solver,whuaegeanse/ceres-solver,kartikkumar/ceres-solver,DougFirErickson/ceres-solver,entylop/ceres-solver,kartikkumar/ceres-solver,stonier/ceres_solver,whuaegeanse/ceres-solver,entylop/ceres-solver