Refresh README to include attribution for external code added after 2017

[spencerkclark]

The README currently only makes reference to derivative code from the "baseline 2017 GFS," but a fair number of substantive updates have been made since then, including the incorporation of:

• SAS convection schemes from GFSv16
• TKE-EDMF scheme from EMC
• Noah-MP scheme from EMC (from NCAR/noahmp, NCAR/ccpp-physics, and ufs-community/ccpp-physics)

It would be good to refresh the README to include attribution for these updates. This issue is meant to track discussion started in #42, and collect any further details of the provenance of these schemes we would like to include the README.

cc: @lharris4 @linjiongzhou @gaokun227 @kaiyuan-cheng