Update CONTRIBUTING.md

Adding information to not add the screenshot to git.

CONTRIBUTING.md

@@ -30,7 +30,7 @@ If your plugin is complex, make sure to include tests, we suggest using [fish-sp
 
 ### Themes
 
-Make sure to include a screenshot in your pull request. A nifty way is too post a comment with the image and link directly to it.
+Make sure to include a screenshot in your pull request, but don't commit the file to git. A nifty way is too post a comment with the image and link directly to it.
 
 [fish-spec]: https://github.com/bpinto/oh-my-fish/tree/master/plugins/fish-spec
 [commit]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html