Clarify ARM support limitation in README

[PallHaraldsson]

Seemingly works now on 64-bt ARM at least on macOS #55

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.89%. Comparing base (b38f02f) to head (293b8bd).
⚠️ Report is 31 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #99       +/-   ##
===========================================
+ Coverage   76.43%   87.89%   +11.46%     
===========================================
  Files           3        3               
  Lines         348      314       -34     
===========================================
+ Hits          266      276       +10     
+ Misses         82       38       -44     

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