Problem: contributing guidelines unclear for new devs

Solution: port Github PR template from CZMQ

.github/PULL_REQUEST_TEMPLATE.md

+# Pull Request Notice
+
+Before sending a pull request make sure each commit solves one clear, minimal,
+plausible problem. Further each commit should have the following format:
+
+```
+Problem: X is broken
+
+Solution: do Y and Z to fix X
+```
+
+Please avoid sending a pull request with recursive merge nodes, as they
+are impossible to fix once merged. Please rebase your branch on
+zeromq/libzmq master instead of merging it.
+
+git remote add upstream git@github.com:zeromq/libzmq.git
+git fetch upstream
+git rebase upstream/master
+git push -f
+
+In case you already merged instead of rebasing you can drop the merge commit.
+
+git rebase -i HEAD~10
+
+Now, find your merge commit and mark it as drop and save. Finally rebase!
+
+If you are a new contributor please have a look at our contributing guidelines:
+[CONTRIBUTING](http://zeromq.org/docs:contributing)