Add protobuf to requirements. (#413)

Idea is to avoid issues like #412 in future.

Author: nwbirnie

README.md

@@ -13,6 +13,11 @@ This project hosts the Java client library for the Google Ads API.
   * Java 1.8+
   * Maven 3.0+
 
+## Compatible dependencies
+
+  * [Protobuf](https://github.com/protocolbuffers/protobuf) 3.12.2+
+  * [Guava](https://github.com/google/guava) 30.0+
+
 ## Maven artifacts
 
     <dependency>