Added PR template

Author: nhaarman

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,5 @@
+Thank you for submitting a pull request! But first:
+
+ - [ ] Can you back your code up with tests?
+ - [ ] Keep your commits clean: [squash your commits if necessary](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History).
+ - [ ] Make sure you're targeting the `dev` branch with the PR.