[docs] update README with simple instructions

README.md

@@ -2,11 +2,65 @@
 
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-## Testing
+## Documentation
+
+You can find the latest documentation in the Datadog's [private repository][docs]
+
+[docs]: http://gems.datadoghq.com/trace/docs/
+
+## Getting started
+
+### Install
+
+If you're using ``Bundler``, just update your ``Gemfile`` as follows:
+
+    source 'https://rubygems.org'
+
+    # tracing gem
+    gem 'ddtrace', :source => 'http://gems.datadoghq.com/trace/'
+
+### Quickstart (manual instrumentation)
+
+If you aren't using a supported framework instrumentation, you may want to to manually instrument your code.
+Adding tracing to your code is very simple. As an example, let’s imagine we have a web server and we want
+to trace requests to the home page:
+
+```
+    require 'ddtrace'
+    require 'sinatra'
+    require 'activerecord'
+
+    # a generic tracer that you can use across your application
+    tracer = Datadog.tracer
+
+    get '/' do
+      tracer.trace('web.request') do |span|
+        # set some span metadata
+        span.service = 'my-web-site'
+        span.resource = '/'
+        span.set_tag('http.method', request.request_method)
+
+        # trace the activerecord call
+        tracer.trace('posts.fetch') do
+          @posts = Posts.order(created_at: :desc).limit(10)
+        end
+
+        # trace the template rendering
+        tracer.trace('template.render') do
+          erb :index
+        end
+      end
+    end
+```
+
+## Development
+
+### Testing
 
 You can launch all tests using the following rake command:
 ```
   $ rake test                     # tracer tests
+  $ appraisal rails-3 rake rails  # rails 3 integration tests
   $ appraisal rails-4 rake rails  # rails 4 integration tests
   $ appraisal rails-5 rake rails  # rails 5 integration tests
   $ appraisal rake rails          # tests for all rails versions