Document commit message syntax, job directives, etc.  Decorate as a cross-reference page to encourage reuse

Change-Id: I3f3b12cfd6cd3dd10a91b28a36f4e487fa25f464
diff --git a/howto/code/index.rst b/howto/code/index.rst
index 685dc2a..998109c 100644
--- a/howto/code/index.rst
+++ b/howto/code/index.rst
@@ -4,6 +4,7 @@
 .. toctree::
    :maxdepth: 2
 
+   commit-message
    job-failures
    linting
    pull_request