Add contributing instructions for gh users (#10913)

* Add contributing instructions for gh users

Pull requests generated from `gh` erase the standard contributing template.

* Update CONTRIBUTING.md

Grammar

Co-authored-by: brandon s allbery kf8nh <allbery.b@gmail.com>

---------

Co-authored-by: brandon s allbery kf8nh <allbery.b@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

CONTRIBUTING.md

@@ -375,9 +375,22 @@ and a feature implementation should go in two different pull requests).
 This is *especially* important when we decide to backport a pull request
 (be it fix or a feature).
 
-Thorough reviews mean less regressions, keeping your pull requests small
+Thorough reviews mean fewer regressions, keeping your pull requests small
 will improve Cabal codebase quality.
 
+### Pull requests for `gh` users
+
+Are you a [`gh`](https://cli.github.com/) (GitHub’s official command line tool)
+user? Input this command to create a pull request:
+
+```
+gh pr create --template pull_request_template.md
+```
+
+This way you will not erase the
+[PR template](https://github.com/haskell/cabal/blob/master/.github/pull_request_template.md)
+all contributors use.
+
 ## Changelog
 
 Anything that changes `cabal-install:exe:cabal` or changes exports from library