add note on updating public key ring

docs/contact/public-keys.md

@@ -79,6 +79,21 @@ gpg --no-default-keyring --keyring ./teampubkeys.gpg --armour \
     The use of `--no-default-keyring` helps ensure that the key you use for a
     recipient is one from our team keyring and not a personal key.
 
+## Updating the public key ring
+
+If you're a DevOps division member, you can open a merge request on the
+[guidebook
+project](https://gitlab.developers.cam.ac.uk/uis/devops/docs/guidebook/) in
+GitLab updating `docs/downloads/teampubkeys.gpg`. To import your _public_ key
+into the keyring:
+
+```sh
+gpg --no-default-keyring --keyring ./teampubkeys.gpg --import [KEYFILE]
+```
+
+where `[KEYFILE]` is your public key. The GPG manual has [a page on extracting
+your public key](https://www.gnupg.org/gph/en/manual/x56.html).
+
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   We vendor our own OpenPGP.js copy to ensure we know the precise version.
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