project.config: add check for Signed-off-by footer

The Go project doesn't use Signed-off-by footers in commit messages.
This is documented at https://go.dev/wiki/CommitMessage. Occasional
contributors will sometimes miss this and include it anyway.

Add a submit requirement that catches accidental uses of such footers
so that this isn't something reviewers need to manually pay attention
to when reviewing CLs.

Change-Id: I0250c6336806ef8eea86fbd88084c615d5f0c54d
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/584938
Reviewed-by: Ian Lance Taylor <iant@google.com>

project.config

diff --git a/project.config b/project.config
index 594e7ca..9721b14 100644
--- a/project.config
+++ b/project.config
@@ -226,3 +226,7 @@
 	applicableIf = branch:\"^refs/heads/release-branch.+\" -prefixsubject:\"[release-branch\"
 	submittableIf = is:false
 	canOverrideInChildProjects = true
+[submit-requirement "No-Signed-off-by-Footer"]
+	description = Changes with a Signed-off-by footer are not submittable. See https://go.dev/wiki/CommitMessage.
+	applicableIf = hasfooter:Signed-off-by
+	submittableIf = is:false