Added usage for login_hint parameter

Author: synth

README.md

@@ -21,12 +21,19 @@ Or install it yourself as:
 
 ## Usage
 
+#### Configuration
 ```ruby
 Rails.application.config.middleware.use OmniAuth::Builder do
   provider :microsoft_graph, ENV['AZURE_APPLICATION_CLIENT_ID'], ENV['AZURE_APPLICATION_CLIENT_SECRET']
 end
 ```
 
+#### Login Hint
+Just add {login_hint: "email@example.com"} to your url generation to form:
+```ruby
+/auth/microsoft_graph?login_hint=email@example.com
+```
+
 ## Contributing
 
 1. Fork it ( https://github.com/synth/omniauth-microsoft_graph/fork )