Add pull request template

Relates elastic#119

Author: dliappis

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,12 @@
+<!--
+Here are a few simple things to check before submitting your pull request
+that can help with the review process. You should delete these items
+from your submission, but they are here to help bring them to your
+attention.
+-->
+
+* For new features, have you added tests?
+* Have you run `make test` and ensured it finishes without errors?
+* Have you executed the `nightly` and `release` [Vagrant workflow](https://github.com/elastic/night-rally/blob/master/README.md#developing-night-rally--rally) successfully?
+* Did you choose a [descriptive title and description](https://chris.beams.io/posts/git-commit/) for your PR?
+* Did you apply appropriate labels?