chore: Update PULL_REQUEST_TEMPLATE.md

better instructions for MDN

Author: marcoscaceres

.github/PULL_REQUEST_TEMPLATE.md

@@ -14,3 +14,5 @@ Documentation (new feature):
 
  * [ ] Pinged MDN
  * [ ] Added example to README or spec
+
+For documentation, either create an [issue](https://github.com/mdn/content/issues) or pull request in [MDN's Content](https://github.com/mdn/content) repo  - providing as much information as you can. PR is prefered.