Mention required dependencies in Readme

Importing annotations.proto will produce a dependency in the generated stubs. This requires the relevant import when using the stubs even if the annotations are not actually used at all by anything but the grpc-gateway plugin.

README.md

@@ -113,7 +113,9 @@ Make sure that your `$GOPATH/bin` is in your `$PATH`.
        --grpc-ruby_out=. \
        path/to/your/service.proto
      ```
-   2. Implement your service
+   2. Add the googleapis-common-protos gem (or your language equivalent) as a dependency to your project.
+   3. Implement your service
+
 5. Generate reverse-proxy
 
    ```sh