Update checking-for-existing-gpg-keys.md

Add a workaround for the “No valid identities found in this GPG data” error.

content/authentication/managing-commit-signature-verification/checking-for-existing-gpg-keys.md

@@ -29,6 +29,7 @@ shortTitle: Existing GPG keys
       $ gpg --armor --export 3AA5C34371567BD2
       # Prints the GPG public key, in ASCII armor format
       ```
+> If you encounter "No valid identities found in this GPG data" error, it may be because the public key you exported is too large or contains useless information, please add GPG option `--export-options export-minimal` to export again, e.g.: `$ gpg --armor --export-options export-minimal --export 3AA5C34371567BD2`
 
       You can then [add your GPG key to your GitHub account](/authentication/managing-commit-signature-verification/adding-a-gpg-key-to-your-github-account).