docs/grpc: added instructions for writing gRPC clients in Ruby

This commit adds a basic set of instructions for installing the necessary
gRPC dependencies to be able to drive `lnd` using Ruby.

docs/grpc/ruby.md

@@ -0,0 +1,106 @@
+# How to write a Ruby gRPC client for the Lightning Network Daemon
+
+This section enumerates what you need to do to write a client that communicates
+with `lnd` in Ruby.
+
+### Introduction
+
+`lnd` uses the `gRPC` protocol for communication with clients like `lncli`.
+
+`gRPC` is based on protocol buffers and as such, you will need to compile
+the `lnd` proto file in Ruby before you can use it to communicate with `lnd`.
+
+### Setup
+
+Install gRPC rubygems:
+
+```
+$ gem install grpc
+$ gem install grpc-tools
+```
+
+Clone the Google APIs repository:
+
+```
+$ git clone https://github.com/googleapis/googleapis.git
+```
+
+Fetch the `rpc.proto` file (or copy it from your local source directory):
+
+```
+$ curl -o rpc.proto -s https://raw.githubusercontent.com/lightningnetwork/lnd/master/lnrpc/rpc.proto
+```
+
+Compile the proto file:
+
+```
+$ grpc_tools_ruby_protoc --proto_path googleapis:. --ruby_out=. --grpc_out=. rpc.proto
+```
+
+Two files will be generated in the current directory: 
+
+* `rpc_pb.rb`
+* `rpc_services_pb.rb`
+
+### Example - Simple client to display wallet balance
+
+Everytime you use the Ruby gRPC you need to require the `rpc_services_pb` file.
+
+We assume that `lnd` runs on the default `localhost:10009`.
+
+We further assume you run `lnd` with `--no-macaroons`.
+
+```ruby
+#!/usr/bin/env ruby
+
+$:.unshift(File.dirname(__FILE__))
+
+require 'grpc'
+require 'rpc_services_pb'
+
+certificate = File.read(File.expand_path("~/.lnd/tls.cert"))
+credentials = GRPC::Core::ChannelCredentials.new(certificate)
+stub = Lnrpc::Lightning::Stub.new('127.0.0.1:10009', credentials)
+
+response = stub.wallet_balance(Lnrpc::WalletBalanceRequest.new())
+puts "Total balance: #{response.total_balance}"
+```
+
+This will show the `total_balance` of the wallet.
+
+#### Example - Streaming client for invoice payment updates
+
+```ruby
+#!/usr/bin/env ruby
+
+$:.unshift(File.dirname(__FILE__))
+
+require 'grpc'
+require 'rpc_services_pb'
+
+certificate = File.read(File.expand_path("~/.lnd/tls.cert"))
+credentials = GRPC::Core::ChannelCredentials.new(certificate)
+stub = Lnrpc::Lightning::Stub.new('127.0.0.1:10009', credentials)
+
+stub.subscribe_invoices(Lnrpc::InvoiceSubscription.new) do |invoice|
+  puts invoice.inspect
+end
+```
+
+Now, create an invoice on your node:
+
+```bash
+$ lncli addinvoice --value=590
+{
+	"r_hash": <R_HASH>,
+	"pay_req": <PAY_REQ>
+}
+```
+
+Next send a payment to it from another node:
+
+```
+$ lncli sendpayment --pay_req=<PAY_REQ>
+```
+
+You should now see the details of the settled invoice appear.