README.md: Add CertificateFile to SSH example config

Otherwise the key ordering is based on agent ordering. With 56152e5da
the certificate will be added at the end of the list, with the Security
Key at the front. This means the certificate will not actually preferred
and you have to touch the key to login.

Author: pkern

README.md

@@ -57,6 +57,7 @@ circumstances:
 $ cat >> ~/.ssh/config
 Match host *.example.com
         IdentitiesOnly yes
+        CertificateFile /run/user/%i/sshca/example-cert.pub
         IdentityFile /run/user/%i/sshca/example
         IdentityFile ~/.ssh/id_ecdsa_sk
 ```