Add instructions for building within CESM, linking to FTorch, and running.

[jatkinson1000]

Closes #6 Relates to #1, #2

I have added instructions for

• [x] obtaining CESM and checking out the relevant tag (2.1.5 tbc with Joan/Qiang)
• [x] pointing to use this fork of CAM as part of CESM build
• [x] pointing to the ICCS fork of CIME as part of the CESM build
  • [x] minimal instructions/notes about building FTorch
• [x] Building and running
  • [x] I assume #4 will be merged so make reference to setting the namelist variables we define there (switch and location of model)

Possible TODO:

• [ ] Specify which variables to add to the fincl files.\ Though perhaps this will come in a later PR as we modify the code and add these?