Warning about editing docs from others' repos

[SusanBrown]

Tell the user, when they open a doc another user's repo, that they need to be added as a collaborator in order to save back to the repo:

[slightly revised wording from James' suggestion]

‘You may edit this document, but changes cannot be saved to the original repository. Git-Writer will create a new repository owned by you, to which you can save a changed version. To change the source document, you would need repository owner SusanBrown[link to github account] to add you as a collaborator to [link to repository].’