GitHub linguist: Mark .rst files as detectable

Linguist excludes documentation by default, which is not so useful on a
repo dedicated to documentation.
This attribute forces it to take .rst files into account and properly
report this report as being 99% reStructuredText.

.gitattributes

@@ -0,0 +1,2 @@
+# Properly detect languages on GitHub
+*.rst linguist-detectable=true