Add README for verify-commits

Author: petertodd

contrib/verify-commits/README.md

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+Tooling for verification of PGP signed commits
+----------------------------------------------
+
+This is an incomplete work in progress, but currently includes a pre-push hook
+script (`pre-push-hook.sh`) for maintainers to ensure that their own commits
+are PGP signed (nearly always merge commits), as well as a script to verify
+commits against a trusted keys list.
+
+
+Using verify-commits.sh safely
+------------------------------
+
+Remember that you can't use an untrusted script to verify itself. This means
+that checking out code, then running `verify-commits.sh` against `HEAD` is
+_not_ safe, because the version of `verify-commits.sh` that you just ran could
+be backdoored. Instead, you need to use a trusted version of verify-commits
+prior to checkout to make sure you're checking out only code signed by trusted
+keys:
+
+    git fetch origin && \
+      ./contrib/verify-commits/verify-commits.sh origin/master && \
+      git checkout origin/master
+
+Note that the above isn't a good UI/UX yet, and needs significant improvements
+to make it more convenient and reduce the chance of errors; pull-reqs
+improving this process would be much appreciated.