[WIP] Document Twisted support

[altendky]

https://twistedmatrix.com/trac/ticket/9815 https://github.com/twisted/twisted/pull/1263

WIP for:

• [ ] Twisted support getting merged
• [ ] Twisted getting released with support

[codecov[bot]]

Codecov Report

Merging #13 (1a1459f) into master (0cfdab8) will not change coverage. The diff coverage is n/a.

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	Coverage Δ
sniffio/_impl.py	90.90% <ø> (ø)