Update pull request message to mention the need for linting and tests/docs

Also remove Markdown formatting for header / links (because the user usually won't see the formatted version anyhow)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,8 @@
-### Thanks for contributing to Wagtail! 🎉
+Thanks for contributing to Wagtail! 🎉
 
-Please review [the contributor guidelines](http://docs.wagtail.io/en/latest/contributing/index.html) and confirm that [the tests pass](http://docs.wagtail.io/en/latest/contributing/developing.html#testing) in case of Python changes.
+Before submitting, please review the contributor guidelines <http://docs.wagtail.io/en/latest/contributing/index.html> and check the following:
+
+* Do the tests still pass? (http://docs.wagtail.io/en/latest/contributing/developing.html#testing)
+* Does the code comply with the style guide? (Run `make lint` from the Wagtail root)
+* For Python changes: Have you added tests to cover the new/fixed behaviour?
+* For new features: Has the documentation been updated accordingly?