📝 added note to 3.0.0 wiki page #474

I created a wiki page
https://github.com/nlohmann/json/wiki/Road-toward-3.0.0 to describe the
transition toward version 3.0.0. On this page, all API-breaking changes
shall be documented.

README.md

@@ -65,6 +65,8 @@ to the files you want to use JSON objects. That's it. Do not forget to set the n
 
 :beer: If you are using OS X and [Homebrew](http://brew.sh), just type `brew tap nlohmann/json` and `brew install nlohmann_json` and you're set. If you want the bleeding edge rather than the latest release, use `brew install nlohmann_json --HEAD`.
 
+:warning: [Version 3.0.0](https://github.com/nlohmann/json/wiki/Road-toward-3.0.0) is currently under development. Branch `develop` is used for the ongoing work and is probably **unstable**. Please use the `master` branch for the last stable version 2.1.1.
+
 
 ## Examples