Update readme

README.md

 # Unity Scriptable Render Loop testbed
 
-> **NOTE**: this is a testbed for a Unity feature that has not shipped yet! The project will not work with any public
-> Unity version, and things in it might and will be broken.
+**NOTE**: this is a testbed for a Unity feature that has not shipped yet! The project will not work with any public
+Unity version, and things in it might and will be broken.
+
+There's a more detailed overview document here: [ScriptableRenderLoop google doc](https://docs.google.com/document/d/1e2jkr_-v5iaZRuHdnMrSv978LuJKYZhsIYnrDkNAuvQ/edit?usp=sharing)
+
+Did we mention it's a very WIP, no promises, may or might not ship feature, anything and everything in it can change? It totally is.