Add ruby example

server/ruby/Gemfile

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+source 'https://rubygems.org'
+
+gem 'mercadopago-sdk'
+gem 'sinatra'

server/ruby/README.md

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+# Checkout payment processing with Checkout Pro
+
+## Using a Ruby server with Sinatra
+
+### Requirements
+- Ruby 2.3.0 or higher
+- Read our [testing instructions](https://www.mercadopago.com/developers/en/guides/online-payments/checkout-pro/test-integration)
+- Setup your credentials: 
+  - `PUBLIC_KEY`and `LOCALE` on client-side `index.js`
+  - Private `ACCESS_TOKEN` on server-side `server.rb`
+
+### How to run it
+- `bundle`
+- `ruby server.rb`
+- Navigate to http://localhost:8080 on your browser

server/ruby/server.rb

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+require 'mercadopago'
+require 'sinatra'
+
+ACCESS_TOKEN = 'YOUR_ACCESS_TOKEN'
+mercadopago = Mercadopago::SDK.new(ACCESS_TOKEN)
+
+post '/create_preference' do
+  payload = JSON.parse(request.body.read)
+
+  preference = {
+    items: [{
+      title: payload['title'],
+      unit_price: payload['price'].to_f,
+      quantity: payload['quantity'].to_f
+    }],
+    back_urls: {
+      success: 'localhost:8080/feedback',
+      failure: 'localhost:8080/feedback',
+      pending: 'localhost:8080/feedback'
+    },
+    auto_return: 'approved'
+  }
+
+  begin
+    response = mercadopago.preference.create(preference)
+
+    { id: response[:response]['id'] }.to_json
+  rescue StandardError => e
+    puts e
+  end
+end
+
+get '/feedback' do
+  params = request.params
+  {
+    Payment: params['payment_id'],
+    Status: params['status'],
+    MechantOrder: params['merchant_order_id']
+  }.to_json
+end
+
+get '/' do
+  send_file '../../client/index.html'
+end
+
+set :public_folder, '../../client'
+set :views, '../../client'
+set :static, true
+set :static_cache_control, [:public, { max_age: 0 }]
+set :port, 8080