Add wiki page to mimic JWQL for contributing

[aliciacanipe]

It might be nice to have our own Wiki with detailed pull request instructions for our notebook submitters like the JWQL one? https://github.com/spacetelescope/jwql/wiki/git-&-GitHub-workflow-for-contributing

@cracraft I'll assign this "to-do" issue to myself.

[cracraft]

James Davies suggested this as the syntax for creating a new branch:
Always use this workflow to set up new branch, rather than what was suggested in jwql wiki.

git fetch upstream
git checkout -b myfeature upstream/master

So when we set up the page, we may want to alter the directions. I followed the instructions on the page and got my fork messed up when I forgot to switch to the branch I'd created and made a push from master.