githooks: add docs prefix

In the process, update the README to point to how to add the githooks
folder. I had to go to aports to find the command.

Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7077>

.githooks/prepare-commit-msg

@@ -49,6 +49,12 @@ case "$lcp" in
 	device/*/*)
 		prefix="$(echo "$lcp" | cut -d/ -f3)"
 		;;
+	docs/*.adoc)
+		prefix="docs/$(basename $lcp .adoc)"
+		;;
+	docs/*)
+		prefix="docs"
+		;;
 	device/*)
 		# Multiple packages touched for same device category. More
 		# logic could be added to detect if they all belong to same