Update README.md

Update readme since the Gitlab provider was added in #286

README.md

@@ -9,7 +9,7 @@
 
 ## Introduction
 
-Laravel Socialite provides an expressive, fluent interface to OAuth authentication with Facebook, Twitter, Google, LinkedIn, GitHub and Bitbucket. It handles almost all of the boilerplate social authentication code you are dreading writing.
+Laravel Socialite provides an expressive, fluent interface to OAuth authentication with Facebook, Twitter, Google, LinkedIn, GitHub, GitLab and Bitbucket. It handles almost all of the boilerplate social authentication code you are dreading writing.
 
 **We are not accepting new adapters.**