Remove the restriction on multi-user PRs

This came up in #docker-dev today.  We're not sure exactly why the original restriction was put it place.  It is not well obeyed.  Please comment if you know the origin of the restriction.

Docker-DCO-1.1-Signed-off-by: Timothy Hobbs <timothyhobbs@seznam.cz> (github: timthelion)

CONTRIBUTING.md

@@ -120,8 +120,6 @@ committing your changes. Most editors have plug-ins that do this automatically.
 Pull requests descriptions should be as clear as possible and include a
 reference to all the issues that they address.
 
-Pull requests must not contain commits from other users or branches.
-
 Commit messages must start with a capitalized and short summary (max. 50
 chars) written in the imperative, followed by an optional, more detailed
 explanatory text which is separated from the summary by an empty line.
@@ -132,6 +130,12 @@ sure to post a comment after pushing. The new commits will show up in the pull
 request automatically, but the reviewers will not be notified unless you
 comment.
 
+Pull requests must be cleanly rebased ontop of master without multiple branches
+mixed into the PR.
+
+**Git tip**: If your PR no longer merges cleanly, use `rebase master` in your
+feature branch to update your pull request rather than `merge master`.
+
 Before the pull request is merged, make sure that you squash your commits into
 logical units of work using `git rebase -i` and `git push -f`. After every
 commit the test suite should be passing. Include documentation changes in the