Show required features in the docs on docs.rs

[Mingun]

Currently, this is nightly-only feature.

See https://stackoverflow.com/questions/61417452

[codecov-commenter]

Codecov Report

Merging #432 (51f5b26) into master (b456b5a) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #432      +/-   ##
==========================================
- Coverage   49.51%   49.50%   -0.01%     
==========================================
  Files          22       22              
  Lines       13847    13847              
==========================================
- Hits         6856     6855       -1     
- Misses       6991     6992       +1     

Flag	Coverage Δ
unittests	49.50% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/lib.rs	12.33% <0.00%> (ø)
src/de/map.rs	72.09% <0.00%> (-0.47%)	⬇️
src/de/mod.rs	77.77% <0.00%> (-0.10%)	⬇️
src/reader.rs	60.41% <0.00%> (+0.05%)	⬆️

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