docs/grpc: add instructions for writing gRPC clients in node.js

This commit adds a basic set of instructions for installing the necessary
gRPC dependencies to be able to drive `lnd` from node.js. In the future,
this section will be expanded with additional example
scripts/applications.

docs/grpc/javascript.md

@@ -0,0 +1,62 @@
+# How to write a simple lnd client in Javascript using nodejs?
+
+First, you need to initialize a simple nodejs project:
+
+```
+npm init (or npm init -f if you want to use the default values without prompt)
+```
+
+Then you need to install the Javascript grpc library dependency:
+
+```
+npm install grpc --save
+```
+
+You also need to copy the `lnd` `rpc.proto` file in your project directory (or at least somewhere reachable by your Javascript code).
+
+The `rpc.proto` file is located in the `lnrpc` directory of the `lnd` sources.
+
+I had to comment the following line in the `rpc.proto` file otherwise `grpc` would crash:
+
+```
+//import "google/api/annotations.proto";
+```
+
+Let's now write some Javascript code that simply displays the getinfo command result on the console:
+
+```
+var grpc = require('grpc');
+
+var lnrpcDescriptor = grpc.load("rpc.proto");
+
+var lnrpc = lnrpcDescriptor.lnrpc;
+
+var lightning = new lnrpc.Lightning('localhost:10009', grpc.credentials.createInsecure());
+
+lightning.getInfo({}, function(err, response) {
+	console.log('GetInfo:', response);
+});
+
+```
+
+You just have to lauch your newly created Javascript file `getinfo.js` using `nodejs`:
+
+```
+node getinfo.js
+```
+
+You should get something like this in your console:
+
+```
+GetInfo: { identity_pubkey: '03c892e3f3f077ea1e381c081abb36491a2502bc43ed37ffb82e264224f325ff27',
+  alias: '',
+  num_pending_channels: 0,
+  num_active_channels: 0,
+  num_peers: 0,
+  block_height: 1087612,
+  block_hash: '000000000000024b2acc37958b15010057c6abc0a48f83c8dd67034bee2cb823',
+  synced_to_chain: true,
+  testnet: true }
+```
+
+Enjoy!