Discuss consequences of switch to src layout in README.md

[namurphy]

Following up on #2598, this PR adds some notes to README.md that describe what contributors need to know to update to the new src layout.

I'll also expand the information in the comment that gets posted to new PRs.

[pheuer]

Apparently you can also do announcement banners for organizations?

https://docs.github.com/en/enterprise-server@3.12/organizations/managing-organization-settings/creating-an-announcement-banner-for-your-organization

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.18%. Comparing base (2a6cf7f) to head (53bf080).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2633   +/-   ##
=======================================
  Coverage   95.18%   95.18%           
=======================================
  Files         103      103           
  Lines        9412     9412           
  Branches     2153     2153           
=======================================
  Hits         8959     8959           
  Misses        276      276           
  Partials      177      177           

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[namurphy]

Apparently you can also do announcement banners for organizations?

Holy hopping rutabagas! I did not know that! I'll try it out.

[namurphy]

Alas, it turns out that you have to have an enterprise GitHub account to be able to do the organization-wide announcements...

[ejohnson-96]

👍