Add warning that docs site is for 3.x, fixes #1679

README.md

@@ -3,6 +3,8 @@
 [![Total Downloads](https://img.shields.io/packagist/dt/monolog/monolog.svg)](https://packagist.org/packages/monolog/monolog)
 [![Latest Stable Version](https://img.shields.io/packagist/v/monolog/monolog.svg)](https://packagist.org/packages/monolog/monolog)
 
+> ⚠ This is the **documentation for Monolog 3.x**, if you are using older releases
+> see the documentation for [Monolog 2.x](https://github.com/Seldaek/monolog/blob/2.x/README.md) or [Monolog 1.x](https://github.com/Seldaek/monolog/blob/1.x/README.md) ⚠
 
 Monolog sends your logs to files, sockets, inboxes, databases and various
 web services. See the complete list of handlers below. Special handlers