Add link to STScI notebook style guide in the contributing docs

[eblur]

Add a link to the Space Telescope notebook style guide into Contributing.md

https://github.com/spacetelescope/style-guides/blob/master/guides/jupyter-notebooks.md

[agarwalrounak]

HI @eblur! I guess this issue is identical to #284 and PR #326 solves this issue as well. I have updated the PR definition accordingly. You can close this.