Merge pull request #304 from Johanna-hub/template

chore:add PULL_REQUEST_TEMPLATE.md

Author: martinthomson

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,12 @@
+Closes #???
+
+The following tasks have been completed:
+
+ * [ ] Confirmed there are no ReSpec/BikeShed errors or warnings.
+ * [ ] Modified Web platform tests (link to pull request)
+
+Implementation commitment:
+
+ * [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
+ * [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
+ * [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)