github/linguist — pull request #1840 by sandyarmstrong (2014-12-06 19:07:47+00:00)
Title: Treat .sketchcs/.sketchfs files as markdown

Xamarin sketch files are interactive coding documents for C# and F#,
serialized as markdown files.

An example is available here: https://github.com/xamarin/sketches/blob/monodevelop-5.7-branch/Basics/basics.sketchcs