(DOCSP-24466): Add link to external review guidelines to PR template (#1630)

Author: jeff-allen-mongo

.github/pull_request_template.md

@@ -11,3 +11,7 @@
 - [ ] Is this free of grammatical errors?
 - [ ] Is this free of staging / rendering issues?
 - [ ] Are all the links working?
+
+## External Review Requirements
+
+[What's expected of an external reviewer?](https://wiki.corp.mongodb.com/display/DE/Reviewing+Guidelines+for+the+MongoDB+Server+Documentation)