doc: Update the README.md

`protoc-gen-go` is now optional, and most people should no longer need
it, unless they are the CI robot, or Jernej.

README.md

@@ -68,19 +68,6 @@ Prerequisites:
     export OASIS_GO=go1.13
     ```
 
-* [protoc-gen-go](https://github.com/golang/protobuf).
-
-  Download and install it with:
-  ```
-  ${OASIS_GO:-go} get github.com/golang/protobuf/protoc-gen-go
-  ```
-
-  _NOTE: If you didn't/can't add `$GOPATH/bin` to your `PATH`, you can install
-  `protoc-gen-go` to `/usr/local/bin` (which is in `$PATH`) with:_
-  ```
-  sudo GOBIN=/usr/local/bin ${OASIS_GO:-go} install github.com/golang/protobuf/protoc-gen-go
-  ```
-
 * [Rust](https://www.rust-lang.org) and the nightly toolchain.
 
   Once you have [`rustup` installed](https://www.rust-lang.org/tools/install),
@@ -104,6 +91,23 @@ Prerequisites:
   cargo install fortanix-sgx-tools sgxs-tools
   ```
 
+* (**OPTIONAL**) [protoc-gen-go](https://github.com/golang/protobuf).
+
+  Download and install it with:
+  ```
+  ${OASIS_GO:-go} get github.com/golang/protobuf/protoc-gen-go
+  ```
+
+  _NOTE: If you didn't/can't add `$GOPATH/bin` to your `PATH`, you can install
+  `protoc-gen-go` to `/usr/local/bin` (which is in `$PATH`) with:_
+  ```
+  sudo GOBIN=/usr/local/bin ${OASIS_GO:-go} install github.com/golang/protobuf/protoc-gen-go
+  ```
+
+  _NOTE: The repository has the most up-to-date files generated by protoc-gen-go
+  committed for convenience.  Installing protoc-gen-go is only required if you
+  are a developer making changes to protobuf definitions used by Go._
+
 In the following instructions, the top-level directory is the directory
 where the code has been checked out.