Incorporate testing documentation from google doc

We currently have a google doc with some testing documentation:

https://docs.google.com/document/d/1oDNFPzw8bFbuyY129vX6-RJG-C9ZNUs-O1x2uVjSO4M/edit

CESM's old README file (from when CESM was in svn) pointed to this with the following text:

```
The most current process for updating and testing CESM versions is documented in a shared
NCAR/UCAR google document at:

https://docs.google.com/a/ucar.edu/document/d/1oDNFPzw8bFbuyY129vX6-RJG-C9ZNUs-O1x2uVjSO4M/edit?usp=sharing
```

I haven't looked through this documentation very carefully. It's quite possible that some of it is out-dated at this point. But some of it may be worth maintaining.

We should consider moving relevant parts to either the Wiki or other documentation within this repository.