Add note about JSHint/Lint to CONTRIBUTING.md

Author: marijnh

CONTRIBUTING.md

@@ -70,3 +70,7 @@ should be asked on the
 - Note that the linter (`bin/lint`) which is run after each commit
   complains about unused variables and functions. Prefix their names
   with an underscore to muffle it.
+
+- CodeMirror does *not* follow JSHint or JSLint prescribed style.
+  Patches that try to 'fix' code to pass one of these linters will be
+  unceremoniously discarded.