Update README.md

Add sentence about the new "zz_" special project branches.

README.md

@@ -4,12 +4,14 @@ CAM code is stored in this repository on branches other than main.  The details
 
 Please see the [wiki](https://github.com/ESCOMP/CAM/wiki) for complete documentation on CAM, getting started with git and how to contribute to CAM's development.
 
-This repository has four branches:
+This repository has four officially-supported branches:
 * main - contains this readme and the Code of Conduct information
 * cam_cesm2_2_rel - contains the current CESM2.2 CAM code
 * cam_cesm2_1_rel - contains the current CESM2.1 CAM code
 * cam_development - contains the current CAM development code (see the important note below before using this branch)
 
+There are other branches in this repo that have a `zz_` prefix which are used for special projects.  Please note that use of these branches is not-supported and may very likely produce un-scientific results in certain configurations.
+
 ## How to checkout and use CAM:
 
 The instructions below assume you have cloned this repository and are in the repository directory. For example: