Add linguist-vendored to show correct language in github

Author: mongo

.gitattributes

@@ -2,3 +2,6 @@ src/*.y linguist-vendored
 src/*.sed linguist-vendored
 src/doc linguist-documentation
 src/*akefile linguist-vendored
+*.png linguist-vendored
+files/* linguist-vendored
+files/*/* linguist-vendored