Update CONTRIBUTING.md with information about using git to make commits

[k4y3ff]

CONTRIBUTING.md currently has a bullet under "Making Changes" that says

(Add more git guidance?)

That line should be replaced with information about how to use git add, git commit, etc. so that students, etc. could potentially submit PRs.

[davidchambers]

I'll work on this if no one beats me to it. :)

[davidchambers]

We can lift bits of ClojureBridge/drawing#16.

[yokolet]

Hi all,
I just updated CONTRIBUTING.md. Since all contents were moved to gh-pages, we need to hit some extra command. Also, I added pictures to clarify what to do on github.

If you guys find something not correct or improvement, make a pull request 😄
Your pull requests also test the updated document.

[yokolet]

Sorry, I hit the wrong button. Absolutely, this discussion is on going.