update issue filing & PR submission instructions

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)

CONTRIBUTING.md

@@ -7,8 +7,10 @@ feels wrong or incomplete.
 ## Reporting Issues
 
 When reporting [issues](https://github.com/dotcloud/docker/issues) 
-on GitHub please include your host OS ( Ubuntu 12.04, Fedora 19, etc... )
-and the output of `docker version` along with the output of `docker info` if possible.  
+on GitHub please include your host OS (Ubuntu 12.04, Fedora 19, etc),
+the output of `uname -a` and the output of `docker version` along with
+the output of `docker info`. Please include the steps required to reproduce
+the problem if possible and applicable.
 This information will help us review and fix your issue faster.
 
 ## Build Environment
@@ -86,6 +88,8 @@ curl -o .git/hooks/pre-commit https://raw.github.com/edsrzf/gofmt-git-hook/maste
 Pull requests descriptions should be as clear as possible and include a
 reference to all the issues that they address.
 
+Pull requests mustn't contain commits from other users or branches.
+
 Code review comments may be added to your pull request. Discuss, then make the
 suggested modifications and push additional commits to your feature branch. Be
 sure to post a comment after pushing. The new commits will show up in the pull