Added a contributing.md file.
Initial draft for contributing.md
Typo
fixes
Contributing.md file done, needs review.

I added my own workflow and feel it would be very nice if others would
do the same.

Maybe the hotfix and release parts could be more documented.

It goes into a lot of details, and can be a starter guide for git and
contributing on git hub for many people that would be interested in a
started site, so it made sense to spend some time on it.
Added a cheat sheet for my workflow.
Merge pull request #160 from andrew13/feature/contributing

Added a contributing.md file