Add a pull request template

Explain that the CSS should be built

.github/pull_request_template.md

@@ -0,0 +1,7 @@
+<!--
+Thanks for contributing to Blacklight!
+
+If you changed any SASS files in this pull-request, ensure you have built the CSS.
+You can do this by running `npm run build` and commit the resulting changes to `app/assets/builds/blacklight.css`
+
+-->