Mention in readme that this is the Yarn v1 site

Author: Daniel15

README.md

@@ -2,7 +2,7 @@ Yarn Website
 [![Netlify Status](https://api.netlify.com/api/v1/badges/85057564-01fa-49d4-b898-30acb74ae19e/deploy-status)](https://app.netlify.com/sites/yarnpkg/deploys)
 ============
 
-This repo contains the source code for the Yarn website.
+This repo contains the source code for the Yarn Classic (v1) website, available at https://classic.yarnpkg.com/. For the Yarn v2 site, see https://github.com/yarnpkg/berry/tree/master/packages/gatsby
 
 In order to get started: