Use grpc-tools npm package in readme

* Make it clearer which directory to `cd` to
* Take advantage of the work done https://github.com/grpc/grpc/issues/5982

examples/node/static_codegen/README.md

@@ -1,7 +1,8 @@
-This is the static code generation variant of the Node examples. Code in these examples is pre-generated using protoc and the Node gRPC protoc plugin, and the generated code can be found in various `*_pb.js` files. The command line sequence for generating those files is as follows (assuming that `protoc` and `grpc_node_plugin` are present, and starting in the base directory of this package):
+This is the static code generation variant of the Node examples. Code in these examples is pre-generated using protoc and the Node gRPC protoc plugin, and the generated code can be found in various `*_pb.js` files. The command line sequence for generating those files is as follows (assuming that `protoc` and `grpc_node_plugin` are present, and starting in the directory which contains this README.md file):
 
 ```sh
-cd ../protos
-protoc --js_out=import_style=commonjs,binary:../node/static_codegen/ --grpc_out=../node/static_codegen --plugin=protoc-gen-grpc=grpc_node_plugin helloworld.proto
-protoc --js_out=import_style=commonjs,binary:../node/static_codegen/route_guide/ --grpc_out=../node/static_codegen/route_guide/ --plugin=protoc-gen-grpc=grpc_node_plugin route_guide.proto
+cd ../../protos
+npm install -g grpc-tools
+grpc_tools_node_protoc --js_out=import_style=commonjs,binary:../node/static_codegen/ --grpc_out=../node/static_codegen --plugin=protoc-gen-grpc=`which grpc_tools_node_protoc_plugin` helloworld.proto
+grpc_tools_node_protoc --js_out=import_style=commonjs,binary:../node/static_codegen/route_guide/ --grpc_out=../node/static_codegen/route_guide/ --plugin=protoc-gen-grpc=`which grpc_tools_node_protoc_plugin` route_guide.proto
 ```