Update README, monkey patch generator.rs

README.md

@@ -12,6 +12,14 @@ And building protos? You use a builder. We don't make some fancy
 structure and give you a marshal function, nah. You give us a blob
 to write data into, and we write what you tell us, no questions asked.
 
+### Design
+
+The `protoc` command generates a CodeGeneratorRequest message; `protoc-gen-roto` (from src/bin/protoc-gen-roto.rs)
+reads this message from stdin, and generated a CodeGeneratorResponse, which it sends to stdout.
+
+The generated files get written to disk by protoc; these should be included in the Rust code being developed to
+use the protobuffers in question.
+
 ### Sample usage
 
 ```rust