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

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
 =======