Add external contributor checklist (#2444)

* Add trivial contribution guidlines

* update toc

* revert trivial guideline, add PR checklist item

* add external contributor checklist

Author: alexandratran

.github/PULL_REQUEST_TEMPLATE.md

@@ -18,3 +18,10 @@ Fixes #
 
 - [ ] If this PR updates or adds documentation content that changes or adds technical meaning, it has received an approval from an engineer or DevRel from the relevant team.
 - [ ] If this PR updates or adds documentation content, it has received an approval from a technical writer.
+
+## External contributor checklist
+
+<!-- If you are an external contributor (outside of the MetaMask organization), complete the following checklist. -->
+
+- [ ] I've read the [contribution guidelines](https://github.com/MetaMask/metamask-docs/blob/main/CONTRIBUTING.md).
+- [ ] I've created a new issue (or assigned myself to an existing issue) describing what this PR addresses.