iCESM1.2 needs user instructions

[nusbaume]

In order for iCESM1.2 to be run with water isotopes or water tracers, specific build and namelist variables must be set, which a new user is unlikely to know about. Thus it would be quite beneficial to add some instructions to this repo, either via the README file, or via some additional text files.

Along with this, it might also make sense to add some example water tracer files, along with additional scripts that generate water isotopic boundary conditions, in case none exist for the model configuration being run by the user.