Added a couple of sentences detailing how we should decide on features CE or EE

Having followed several issues that led to discussions whether a specific functionality needs to be CE or EE I realized our definition on how we decide on features for EE is not specific enough.

The latest issue which was a catalyst in my eyes was https://gitlab.com/gitlab-org/gitlab-ce/issues/4058. I realized that this is exactly the line to follow - if the functionality we're introducing makes using GitLab easier on our core functionality (issues, code review etc.) it should be available to all. I think issue #4058 is a good example of that - making it easier to find other issues and finding duplicates improves on the ease-of-use for Issues.

If we're considering introducing functionality that makes using GitLab more efficient or helps handle complex situations like: handling permissions, merge approvals and reporting that functionality should be considered for EE.

I tried to keep this introduction short and clear but understand it likely needs work on as far as phrasing it properly.