Update README.md with testing instructions

utils/python/MITgcmutils/README.md

@@ -8,3 +8,15 @@ This package is developed as a part of MITgcm on
 
 It is documented in the MITgcm
 [manual](http://mitgcm.rtfd.io/en/latest/utilities/utilities.html#mitgcmutils).
+
+To test any changes to MITgcmutils, do the following in a clone of the MITgcm repository:
+1. Check out the submodule with reference data:
+```
+    git submodule init
+    git submodule update
+```
+3. Install the needed python versions (python3.7 through python3.12).
+4. Install tox: https://tox.wiki/en/4.15.0/installation.html
+   On a mac, it may be necessary to install pipx via macports/brew
+   or use the "pip" method.
+5. Run "tox" in the top MITgcmutils directory.