Mention SSH setup in CONTRIBUTING.md

[GuanyangLuo]

When I tried to run the following commands from the "Git recipes" in CONTRIBUTING.md, I got error messages:

git remote add rlworkgroup git@github.com:rlworkgroup/dowel.git

git reset --hard master rlworkgroup/master

However, the following would work:

git remote add rlworkgroup https://github.com/rlworkgroup/dowel.git

git checkout master
git fetch rlworkgroup
git reset --hard rlworkgroup/master

Should CONTRIBUTING.md be updated?

[krzentner]

Hello and thanks for filing this issue.

The contributing instructions assume you're using ssh to access github.

I think you're right, we should probably update CONTRIBUTING.md to mention that.