readme: Clarify which "determinism" features are supported

[axic]

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[codecov]

Codecov Report

Merging #835 (eee96c3) into master (354e0a2) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head eee96c3 differs from pull request most recent head 6c46bdc. Consider uploading reports for the commit 6c46bdc to get more accurate results

@@            Coverage Diff             @@
##           master     #835      +/-   ##
==========================================
+ Coverage   99.26%   99.27%   +0.01%     
==========================================
  Files          88       88              
  Lines       13268    13296      +28     
==========================================
+ Hits        13170    13200      +30     
+ Misses         98       96       -2     

Flag	Coverage Δ
rust	98.48% <ø> (ø)
spectests	89.92% <ø> (?)
unittests	99.22% <ø> (?)
unittests-32	99.32% <ø> (ø)

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Impacted Files	Coverage Δ
test/unittests/instantiate_test.cpp	100.00% <0.00%> (ø)
test/unittests/floating_point_utils_test.cpp	100.00% <0.00%> (ø)
lib/fizzy/execute.cpp	99.30% <0.00%> (+<0.01%)	⬆️
test/unittests/execute_floating_point_test.cpp	99.71% <0.00%> (+<0.01%)	⬆️
test/testfloat/testfloat.cpp	97.24% <0.00%> (+1.37%)	⬆️