Add pull request template

This commits adds a pull request template for contributors that submit
documentation updates or code patches.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,7 @@
+<!--
+- Have you signed the [contributor license agreement](https://www.elastic.co/contributor-agreement)?
+- Have you followed the [contributor guidelines](https://github.com/elastic/elasticsearch/blob/master/.github/CONTRIBUTING.md)?
+- If submitting code, have you built your formula locally prior to submission with `gradle check`?
+- If submitting code, is your pull request against master? Unless there is a good reason otherwise, we prefer pull requests against master and will backport as needed.
+- If submitting code, have you checked that your submission is for an [OS that we support](https://www.elastic.co/support/matrix#show_os)?
+-->