doc: mention the commit message hook for --signoff

There's a useful section at the end of the doc
to set up a git hook so we don't have to specify -s.
Link to it when -s is first mentioned.

Closes cue-lang#1369.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I155665a95c6612d0ef5efc8745c2d092b2dfd109
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/532889
Reviewed-by: Paul Jolly <paul@myitcv.io>

doc/contribute.md

@@ -180,6 +180,10 @@ When committing, use the `--signoff` (or `-s`) flag:
 git commit -s
 ```
 
+You can also [set up a prepare-commit-msg git
+hook](#do-i-really-have-to-add-the--s-flag-to-each-commit) to not have to supply
+the `-s` flag.
+
 The explanations of the GitHub and GerritHub contribution workflows that follow
 assume all commits you create are signed-off in this way.