Add documentation for two factor authentication

Author: romainneutron

doc/two_factor_authentication.md

@@ -0,0 +1,19 @@
+## Two factor authentication
+[Back to the navigation](index.md)
+
+
+### Raising the exception
+
+```php
+try {
+    $authorization = $github->api('authorizations')->create();
+} catch (Github\Exception\TwoFactorAuthenticationRequiredException $e {
+    echo sprintf("Two factor authentication of type %s is required.", $e->getType());
+}
+```
+
+Once the code has been retrieved (by sms for example), you can create an authorization:
+
+```
+$authorization = $github->api('authorizations')->create(array('note' => 'Optional'), $code);
+```