Link to protoc installation instructions

Fixes #922

README.markdown

@@ -79,7 +79,7 @@ It will also generate client implementations of `PingService`; currently [Twirp]
 - `protoc --plugin=./node_modules/.bin/protoc-gen-ts_proto --ts_proto_out=. ./simple.proto`
   - (Note that the output parameter name, `ts_proto_out`, is named based on the suffix of the plugin's name, i.e. "ts_proto" suffix in the `--plugin=./node_modules/.bin/protoc-gen-ts_proto` parameter becomes the `_out` prefix, per `protoc`'s CLI conventions.)
   - On Windows, use `protoc --plugin=protoc-gen-ts_proto=".\\node_modules\\.bin\\protoc-gen-ts_proto.cmd" --ts_proto_out=. ./simple.proto` (see [#93](https://github.com/stephenh/ts-proto/issues/93))
-  - Ensure you're using a modern `protoc`, i.e. the original `protoc` `3.0.0` doesn't support the `_opt` flag
+  - Ensure you're using a modern `protoc` (see [installation instructions for your platform](https://grpc.io/docs/protoc-installation/), i.e. `protoc` v`3.0.0` doesn't support the `_opt` flag
 
 This will generate `*.ts` source files for the given `*.proto` types.