Clarify that Bonmin and Couenne do not work on Apple aarch64

[odow]

Document #193

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.80%. Comparing base (e32115c) to head (f3cc026).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #194   +/-   ##
=======================================
  Coverage   98.80%   98.80%           
=======================================
  Files           1        1           
  Lines          84       84           
=======================================
  Hits           83       83           
  Misses          1        1           

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