Get linguist grammer to get GitHub syntax highlighting for SDF files

[mithro]

From https://help.github.com/en/articles/creating-and-highlighting-code-blocks#syntax-highlighting

We use Linguist to perform language detection and to select third-party grammars for syntax highlighting. You can find out which keywords are valid in the languages YAML file.

See also https://github.com/github/linguist/blob/master/vendor/README.md