Explain how to use local GPG keys for a repository
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### What needs to get done
[This page](https://documentation.ubuntu.com/snapcraft/stable/reference/package-repositories/) refers to the craft-archives docs for information about adding repositories, but craft-archives makes the local keyring location configurable. We should explain the format of the keyring and where it needs to be stored
### Why it needs to get done
Prevent user confusion
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