Designate our markdown files as our source code

This parallels a change submitted by @i-ky on our learn repo,
as a way to attract the right participation.  Our repo currently
looks more like a JavaScript package, which may partially explain
several recent issues opened with the expectation that this is
a published JavaScript project. Our JavaScript is really just our
test environment for our Markdown.

Author: handrews

.gitattributes

@@ -0,0 +1 @@
+*.md linguist-detectable