Document change ID hook in the README.

Change-Id: I09d72debafa2ff31733f6afa782d5de572def557
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/6940
Commit-Queue: Brett Wilson <brettw@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>

README.md

diff --git a/README.md b/README.md
index 538b34d..1f7f2ff 100644
--- a/README.md
+++ b/README.md
@@ -77,6 +77,10 @@
     git commit
     git push origin HEAD:refs/for/master
 
+The first time you do this you'll get an error from the server about a missing
+change-ID. Follow the directions in the error message to install the change-ID
+hook and run `git commit --amend` to apply the hook to the current commit.
+
 When revising a change, use:
 
     git commit --amend