Document the minimum commit body length gitlint enforces

Gitlint's B5 rule rejects a body under 20 characters. The guidelines listed
the other body rules but not this one, so a short body failed CI with no
documented reason.

Author: Zachary Burnham

.github/commit-guidelines.md

@@ -12,6 +12,7 @@ These align with the gitlint rules run in CI.
 ## Body (optional)
 
 - Blank line after the subject.
+- At least 20 characters, if present.
 - Wrap lines at 80 characters.
 - Explain what and why over how; link issues like "Fixes #1234" when applicable.
 - No hard tabs, no trailing whitespace.