chore: README.md to explain service_config.proto (#1361)

* chore: README.md to explain service_config.proto

* add link

gapic-generator-java/README.md

@@ -6,3 +6,16 @@ Replaces the Java parts of the
 [older monolithic generator](https://github.com/googleapis/gapic-generator).
 
 See [DEVELOPMENT.md](DEVELOPMENT.md) for setting up development environment.
+
+## service_config.proto
+
+We use the `src/main/proto/service_config.proto` file to generate corresponding
+Java class files.
+They are needed to generate client libraries for gRPC-based Google services.
+
+The source of `src/main/proto/service_config.proto` is the
+[https://github.com/grpc/grpc-proto repository](
+https://github.com/grpc/grpc-proto/blob/master/grpc/service_config/service_config.proto).
+We copy the file from the repository when a new enhancement is made in the file
+and the service team asks us to incorporate the enhancement into the code
+generator.