XO Chip Documentation

[craigthomas]

This PR brings in documentation regarding the XO chip specification, as well as a ROM compatibility checklist. This PR also fixes a problem with bit shifting. Unit tests updated to catch new condition. This PR closes #43

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.93%. Comparing base (b802ca0) to head (8f803f7).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   92.87%   94.93%   +2.05%     
==========================================
  Files           4        4              
  Lines         660      691      +31     
==========================================
+ Hits          613      656      +43     
+ Misses         47       35      -12     

Files with missing lines	Coverage Δ
chip8/cpu.py	98.97% <100.00%> (+2.52%)	⬆️

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