Add message about gitbook.com on README (#2170)

SamyPesse · web-flow · commit 6c6ef7f4af32 · 2018-12-27T17:51:39.000+01:00
diff --git a/README.md b/README.md
@@ -1,3 +1,9 @@
+> ## ⚠️ Deprecation warning:
+> As the efforts of the GitBook team are focused on the [GitBook.com](https://www.gitbook.com) platform, the CLI is no longer under active development.  
+> All content supported by the CLI are mostly supported by our [GitBook.com / GitHub integration](https://docs.gitbook.com/integrations/github).  
+> Content hosted on the [legacy.gitbook.com](https://legacy.gitbook.com) will continue working until further notice. For differences with the new vesion, check out our [documentation](https://docs.gitbook.com/v2-changes/important-differences).
+
+
 GitBook
 =======
 
