Add some linguist-detectable attributes (#3806)

This allows to test a use-case where markdown should be reported
in the language stats (e.g because the main purpose of the repo is to
hold prose in markdown format) while HTML should be excluded from the
language stats (e.g because it's example output).

.gitattributes

@@ -0,0 +1,9 @@
+Gemfile linguist-vendored=true
+lib/linguist.rb linguist-language=Java
+test/*.rb linguist-language=Java
+Rakefile linguist-generated
+test/fixtures/* linguist-vendored=false
+README.md linguist-documentation=false
+samples/Arduino/* linguist-documentation
+samples/Markdown/*.md linguist-detectable=true
+samples/HTML/*.html linguist-detectable=false

test/fixtures/foo.rb

@@ -0,0 +1,3 @@
+def foo
+  return "BAR"
+end