README.rst still references manage_externals

[mnlevy1981]

Brief summary of bug

In README.rst, users are still directed to run manage_externals/checkout_externals:

General bug information

CESM version you are using: cesm3.0-alphabranch (as of 397d515)

Machine you are using: Just looking at it on github

Have you modified the code? (If so, please point us to you changes.): No

[mnlevy1981]

The current quick-start is based off old CESM 2.0.1 documentation, so it also mentions manage externals