Add a PR template instructing users to run prePR

Author: japgolly

.github/pull_request_template.md

@@ -0,0 +1,8 @@
+Hi there!
+
+Before submitting a PR containing any Scala changes, please make sure you...
+
+* run `sbt prePR`
+* commit changes to `api-reports`
+
+Thanks for contributing!