Add context to git format-patch instructions of the Quickstart section

[bcr]

Summary

The git format-patch instruction in the Quickstart section of the Primer did not include context. This is useful to have as Phabricator uses this information to add an extra button to comment windows when reviewing. Beginners should add this, so extend the example to include it. Additionally, add an explanatory message at the bottom of the example.

Test Plan

1. Apply the patch
2. Check the output
3. Comment or approve