Add "Sign your PRs" TOC item to contributing.md.
Add "Sign your PRs" TOC item to contributing.md.

Signed-off-by: Elijah Lynn <elynn@redhat.com>

Closes: #954
Approved by: rhatdan

rhatdan/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah-1,projectatomic/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,nalind/buildah,ipbabble/buildah,TomSweeneyRedHat/buildah,ipbabble/buildah,rhatdan/buildah,nalind/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah