Add notes on good cover letters

Cover letters, or PR messages, are very important for "selling" your
changes to the maintainers, and those are ofter overlooked.

Add notes on good cover letters, inspired by kernel development git best practices.

Signed-off-by: Anne Macedo <annie@retpolanne.com>

Author: retpolanne

README.md

@@ -247,6 +247,20 @@ Resolves: #123
 See also: #456, #789
 ```
 
+## Notes on PR messages and cover letters
+
+Most developers open Pull Requests (PR) on a Git repo through a platform (Github, Gitlab),
+while kernel developers  and people who send patches through email ([yes, that's a
+thing](https://git-scm.com/docs/git-send-email/2.45.0) refer to it as "cover letter".
+
+A good cover letter, or PR message, will summarize, give background and context to a series of
+commits that are related. Good writing on this part will help to "sell" your commit series to
+the maintainers of a project, as they'll be able to understand why you are presenting such changes
+together.
+
+If you're writing a smaller (single or a few commits) change, treat the first commit as a cover letter.
+Github, for example, uses this first commit as the default PR message.
+
 ## Rebase vs. Merge
 
 This section is a **TL;DR** of Atlassian's excellent tutorial, ["Merging vs. Rebasing"](https://www.atlassian.com/git/tutorials/merging-vs-rebasing).