Update CONTRIBUTING.md

Author: bep

CONTRIBUTING.md

@@ -93,6 +93,7 @@ Most title/subjects should have a lower-cased prefix with a colon and one whites
 * If this commit touches many packages without a common functional topic, prefix with `all:` (e.g. `all: Reformat Go code`)
 * If this is a documentation update, prefix with `docs:`.
 * If nothing of the above applies, just leave the prefix out.
+* Note that the above excludes nouns seen in other repositories, e.g. "chore:".
 
 Also, if your commit references one or more GitHub issues, always end your commit message body with *See #1234* or *Fixes #1234*.
 Replace *1234* with the GitHub issue ID. The last example will close the issue when the commit is merged into *master*.