Make GitHub recognize the repo sources are largely Markdown (mdn#14123)

* Make GitHub recognize the repo sources are largely Markdown

This change causes GitHub to recognize that the repo sources are
currently 97.1% Markdown and 2.9% HTML.

Otherwise, without this change, the GitHub UI incorrectly shows the
repo sources as being 100% HTML.

See https://github.com/github/linguist/blob/master/docs/overrides.md#detectable

* Exclude README.md etc. from linguist detection

Author: sideshowbarker

.gitattributes

@@ -1 +1,6 @@
 *	text=auto eol=lf
+*.md	linguist-detectable
+CODE_OF_CONDUCT.md	-linguist-detectable
+LICENSE.md	-linguist-detectable
+README.md	-linguist-detectable
+REVIEWING.md	-linguist-detectable