Mention cross-browser testing in PR template

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,4 +5,5 @@ Before submitting, please review the contributor guidelines <http://docs.wagtail
 * 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 front-end changes: Did you test on all of Wagtail’s supported browsers? **Please list the exact versions you tested**.
 * For new features: Has the documentation been updated accordingly?