Add explicit additional step to the readme

This was easy to miss during the process of setting up the action, and it would have been nice for it to be more explicit when we were setting this up.

README.md

@@ -37,8 +37,7 @@ ___
 
 ## Prerequisites
 
-First, [generate a GPG key](https://docs.github.com/en/github/authenticating-to-github/generating-a-new-gpg-key) and
-export the GPG private key as an ASCII armored version to your clipboard:
+First, [generate a GPG key](https://docs.github.com/en/github/authenticating-to-github/generating-a-new-gpg-key) and then [add the public key to your GitHub account](https://docs.github.com/en/authentication/managing-commit-signature-verification/adding-a-gpg-key-to-your-github-account). Once that's done, export the GPG private key as an ASCII armored version to your clipboard:
 
 ```shell
 # macOS