Added example for working with SwiftUI

[MrWoWander]

I suggest adding an example of working with SwiftUI to Down-Example, because people are starting to switch to it and the need is growing (at one time, I myself asked a question about this in the issues 😊)

This example demonstrates working with DownView and with Down.toAttributedString

P.S. - Attribute line down by default uses the standard Styler class, so until PR #258 is adopted no images will be displayed

[codecov]

Codecov Report

Merging #260 (df0f21e) into master (bf24fcb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #260   +/-   ##
=======================================
  Coverage   91.31%   91.31%           
=======================================
  Files          60       60           
  Lines        1059     1059           
=======================================
  Hits          967      967           
  Misses         92       92           

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