Copy mailing list information from docs to readme [ci skip]

[abyrd]

As pointed out by @leonardehrenfried, the update to the docs about the mailing list also needed to be copied into the top-level readme.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.90%. Comparing base (3572d3b) to head (8823782).
Report is 23 commits behind head on dev-2.x.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #5830      +/-   ##
=============================================
+ Coverage      67.88%   67.90%   +0.02%     
+ Complexity     16558    16556       -2     
=============================================
  Files           1908     1910       +2     
  Lines          72355    72436      +81     
  Branches        7443     7447       +4     
=============================================
+ Hits           49117    49190      +73     
- Misses         20716    20725       +9     
+ Partials        2522     2521       -1     

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