[WIP] Document Twisted support

[altendky]

https://twistedmatrix.com/trac/ticket/9815
twisted/twisted#1263

WIP for:

• Twisted support getting merged
• Twisted getting released with support

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.51%. Comparing base (0cfdab8) to head (1a1459f).
Report is 17 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #13   +/-   ##
=======================================
  Coverage   96.51%   96.51%           
=======================================
  Files           4        4           
  Lines          86       86           
  Branches       17       15    -2     
=======================================
  Hits           83       83           
  Misses          2        2           
  Partials        1        1           

Files with missing lines	Coverage Δ
sniffio/_impl.py	90.90% <ø> (ø)

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