More notes on vendored config

lib/linguist/vendor.yml

@@ -1,3 +1,12 @@
+# Vendored files and directories are excluded from language
+# statistics.
+#
+# Lines in this file are Regexps that are matched against the file
+# pathname.
+#
+# Please add additional test coverage to
+# `test/test_blob.rb#test_vendored` if you make any changes.
+
 ## Vendor Conventions ##
 
 # Caches