Add cam_cesm2_2_rel branch info to README file.

Also update relevant tag numbers.

README.md

@@ -4,9 +4,10 @@ CAM code is stored in this repository on branches other than master.  The detail
 
 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 three branches:
+This repository has four branches:
 * master - contains this readme and the Code of Conduct information
-* cam_cesm2_1_release - contains the current CESM2.1 CAM code
+* 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)
 
 ## How to checkout and use CAM:
@@ -16,19 +17,23 @@ The instructions below assume you have cloned this repository and are in the rep
 git clone https://github.com/ESCOMP/CAM
 cd CAM
 ```
-
+### To run CAM compatible with the CESM2.2 release:
+```
+git checkout cam_cesm2_2_rel_03
+./manage_externals/checkout_externals
+```
 ### To run CAM compatible with the CESM2.1 release:
 ```
-git checkout cam_cesm2_1_rel_33
+git checkout cam_cesm2_1_rel_46
 ./manage_externals/checkout_externals
 ```
-### To view the release branch in github, go to the "Branch:master" pulldown menu and select cam_cesm2_1_rel
+### To view the release branches in Github, go to the "Branch:master" pulldown menu and select the appropriate branch.
 
 ### To use unsupported CAM **development** code:
 
 ## NOTE: This is **unsupported** development code and is subject to the [CESM developer's agreement](http://www.cgd.ucar.edu/cseg/development-code.html).
 ```
-git checkout cam6_2_000
+git checkout cam6_3_000
 ./manage_externals/checkout_externals
 ```
 ### CAM Documentation - https://ncar.github.io/CAM/doc/build/html/index.html