Add PR template (#627)

Roughly the same as in the datadog-agent repo

Author: mx-psi

Diff for: .github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,15 @@
+### What does this PR do?
+
+<!--A brief description of the change being made with this pull request.-->
+
+### Motivation
+
+<!--What inspired you to submit this pull request?-->
+
+### Additional Notes
+
+<!--Anything else we should know when reviewing?-->
+
+### Describe your test plan
+
+<!--Write there any instructions and details you may have to test your PR.-->