mockgcp: update readme for renaming of new services

If adding a new API not under google/cloud, we may need a few new sed
lines.

mockgcp/README.md

@@ -32,7 +32,8 @@ Broadly the steps are:
    refer to your resource's API documentation to identify the service name, for example [privateca](https://cloud.google.com/certificate-authority-service/docs/reference/rest#service:-privateca.googleapis.com).
    Once you identify the service, find the proper path to the proto files, for example:
    `cloud/security/privateca/v1/*.proto`. Then replace the prefix `googleapis/google/` to `./third_party/googleapis/mockgcp/`,
-   and add into the Makefile.
+   and add into the Makefile.  If you're adding an API outside of googleapis/google/cloud,
+   you may need to add commands to rename the API o mockgcp in [fixup-third-party.sh](fixup-third-party.sh]
 1. Add a subdirectory called `mock<servicename>`.
 
    Copying one of the existing ones. `mockprivateca` is a reasonable basic one. Keep the files names.go and service.go,