include how to change the global file in development documentation

[jdkent]

Describe the bug
I was trying to patch in my own development version of xcpEngine, but the environment variables were not set correctly (and I didn't track down the error very quickly). They are all redefined in the global file.

Runtime Information
I was using singularity.

Additional context
I think a solution would be to include more information in the development portion of the docs to mention to the user/developer to make sure the environment variables are set correctly.

[a3sha2]

Hi @jdkent
Thanks you James, we will correct it in the documentation.