Top level README is way out of date.

### Brief summary of bug

It refers to a spreadsheet that hasn't been updated since 2018 or so.

### General bug information

**CESM version you are using:** [Output of `git describe`]

**Machine you are using:** [Fill this in]

**Have you modified the code? (If so, please point us to you changes.):** [Yes / No]

### Details of bug

[Fill in details here.]

### Steps to reproduce the problem. Include externals description file(s) and link to public repository

[Fill this in with anything relevant that you haven't already noted; if there is nothing to add, delete this section.]

### Screen output or log file showing the error message and context

[Fill this in with anything relevant that you haven't already noted; if there is nothing to add, delete this section.]