pw_presubmit: Document install_hook

Also provide some advice on what's suitable for use as a git pre-push
hook.

Change-Id: I1c7c05bb33b6841cdc994c6b24fa905cf6487920
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/93602
Pigweed-Auto-Submit: Ted Pudlik <tpudlik@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>

pw_presubmit/docs.rst

@@ -156,6 +156,24 @@ pw_presubmit
 
 .. _example-script:
 
+
+Git hook
+--------
+You can run a presubmit program or step as a `git hook
+<https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks>`_ using
+``pw_presubmit.install_hook``. This can be used to run certain presubmit
+checks before a change is pushed to a remote.
+
+We strongly recommend that you only run fast (< 15 seconds) and trivial checks
+as push hooks, and perform slower or more complex ones in CI. This is because,
+
+* Running slow checks in the push hook will force you to wait longer for
+ ``git push`` to complete, and
+* If your change fails one of the checks at this stage, it will not yet be
+ uploaded to the remote, so you'll have a harder time debugging any failures
+ (sharing the change with your colleagues, linking to it from an issue
+ tracker, etc).
+
 Example
 =======
 A simple example presubmit check script follows. This can be copied-and-pasted