Update CHECKOUT_code (#37)

* Update CHECKOUT_code

* Update README.md as per PR comments

---------

Co-authored-by: Lauren Chilutti <60401591+laurenchilutti@users.noreply.github.com>

Author: kaiyuan-cheng

CHECKOUT_code

@@ -35,8 +35,8 @@ release="main"
 
 fv3_release=$release
 phy_release=$release
-fms_release="2023.02"
-drivers_release="2023.02"
+fms_release="2023.04"
+drivers_release=$release
 
 git clone -b ${fv3_release}   https://github.com/NOAA-GFDL/GFDL_atmos_cubed_sphere
 git clone -b ${phy_release}   https://github.com/NOAA-GFDL/SHiELD_physics

README.md

@@ -10,12 +10,14 @@ The top level directory structure groups source code and input files as follow:
 | --------------       | ------- |
 | ```LICENSE.md```     | copy of the Gnu Lesser General Public license, version 3 |
 | ```README.md```      | this file with basic pointers to more information |
-| ```CHECKOUT_code```  | script to download necessary source for proper build GFDL's SHield and FV3 Solo models |
+| ```CHECKOUT_code```  | script to download necessary source for proper build GFDL's SHield and FV3 Solo models <sup>*</sup>|
 | ```Build/```         | contains scripts used for building models listed above |
 | ```mkmf/```          | submodule entry point for the externally managed [mkmf software](https://github.com/NOAA-GFDL/mkmf) |
 | ```RTS/```           | contains scripts for use in CI software regression testing (see [RTS/README.md](https://github.com/NOAA-GFDL/SHiELD_build/blob/main/RTS/README.md))|
 | ```site/```          | contains site specific scripts and compiler make templates |
 
+<sup>*</sup>By default, ```CHECKOUT_code``` checks out the latest main branch from each repository, which may include experimental features
+
 # Compiling
 
 Be sure to download the mkmf submodule prior to beginning.  To use: