Actually include the PR template

Author: rreusser

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,15 @@
+Thanks for your interest in plotly.js!
+
+Developers are strongly encouraged to first make a PR to their own plotly.js
+fork and ask one of the maintainers to review the modifications there. Once the
+pull request is deemed satisfactory, the developer will be asked to make a pull
+request to the main plotly.js repo and may be asked to squash some commits
+before doing so.
+
+Developers should `git rebase` their local branch off the latest `master` before 
+opening a pull request.
+
+Note that it is forbidden to force push (i.e. `git push -f`) to remote branches
+associated with opened pull requests. Force pushes make it hard for maintainers 
+to keep track of updates. Therefore, if required, please
+`git merge master` into your PR branch instead of `git rebase master`.