fdp-primer: Include context in the git format-patch for users to use

The quick start section did not instruct the reader to include context
in the git format-patch example. This is not ideal as it makes reviewing
easier. Include this information with a bit of explanation below it.

Event: Halifax Hackathon 202606
Location: A different chair than obiwac's
Reviewed by: lwhsu
Differential Revision: https://reviews.freebsd.org/D57891