Update documentation

[devinus]

The doc example has syntax errors and doesn't work with either the Grape gem or master branch. The example needs to mixin a session middleware for Warden to use, and it should look more like this:

use Warden::Manager do |manager|
  Warden::Strategies.add :bearer, Warden::OAuth2::Strategies::Bearer
  Warden::Strategies.add :client, Warden::OAuth2::Strategies::Client
  Warden::Strategies.add :public, Warden::OAuth2::Strategies::Public
  manager.default_strategies :bearer, :client, :public
  manager.failure_app = Warden::OAuth2::FailureApp
end

[aiwilliams]

After running into invalid documentation myself, I came to understand that the code would be like this, since the argument to the block is a Warden::Config:

use Warden::Manager do |config|
  config.strategies.add :bearer, Warden::OAuth2::Strategies::Bearer
  config.strategies.add :client, Warden::OAuth2::Strategies::Client
  config.strategies.add :public, Warden::OAuth2::Strategies::Public
  config.default_strategies :bearer, :client, :public
  config.failure_app = Warden::OAuth2::FailureApp
end