Updated README to better document required tools

README.md

@@ -136,7 +136,7 @@ This runs a multi-process test:
  - A [test](integration/log_integration_test.go) that starts a Trillian server
    in Log mode, together with a signer, logs many leaves, and checks they are
    integrated correctly.
-   
+
 ### Deployment
 
 You can find instructions on how to deploy Trillian in [deployment](/deployment)
@@ -156,7 +156,7 @@ Some of the Trillian Go code is autogenerated from other files:
 
  - [gRPC](http://www.grpc.io/) message structures are originally provided as
    [protocol buffer](https://developers.google.com/protocol-buffers/) message
-   definitions.
+   definitions. See also, https://grpc.io/docs/protoc-installation/.
  - Some unit tests use mock implementations of interfaces; these are created
    from the real implementations by [GoMock](https://github.com/golang/mock).
  - Some enums have string-conversion methods (satisfying the `fmt.Stringer`
@@ -174,11 +174,13 @@ the original files; if you do, you'll need to install the prerequisites:
     cd $(go list -f '{{ .Dir }}' github.com/google/trillian); \
     go install github.com/golang/mock/mockgen; \
     go install google.golang.org/protobuf/proto; \
-    go install google.golang.org/protobuf/protoc-gen-go; \
+    go install google.golang.org/protobuf/cmd/protoc-gen-go; \
+    go install google.golang.org/grpc/cmd/protoc-gen-go-grpc; \
     go install github.com/pseudomuto/protoc-gen-doc/cmd/protoc-gen-doc; \
     go install golang.org/x/tools/cmd/stringer
     ```
   - protocol buffer definitions for standard Google APIs:
+    TODO(#2763): this doesn't need to be in `GOPATH/src`.
 
     ```bash
     git clone https://github.com/googleapis/googleapis.git $(go env GOPATH)/src/github.com/googleapis/googleapis