Explicitly list PolyScope X compatibility

[urfeex]

I tried browsing through the documentation trying to make it clear that PolyScope X is supported by this library.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.76%. Comparing base (276120d) to head (fb40441).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #402      +/-   ##
==========================================
- Coverage   42.90%   42.76%   -0.14%     
==========================================
  Files          99       99              
  Lines        8627     8621       -6     
  Branches     1170     1170              
==========================================
- Hits         3701     3687      -14     
- Misses       4647     4654       +7     
- Partials      279      280       +1     

Flag	Coverage Δ
start_ursim	81.64% <ø> (-2.92%)	⬇️
ur20-latest	40.75% <ø> (-0.02%)	⬇️
ur5-3.14.3	40.54% <ø> (-0.10%)	⬇️
ur5e-10.7.0	35.69% <ø> (-0.11%)	⬇️
ur5e-5.9.4	40.76% <ø> (ø)

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