Merge pull request #1720 from typelevel/fix-334

 type classes with only defined laws to guideline

Author: ceedubs

docs/src/main/tut/guidelines.md

@@ -111,5 +111,11 @@ abstract class KleisliInstance1 {
 }
 ```
 
+### Type classes that ONLY define laws.
+
+We can introduce new type classes for the sake of adding laws that don't apply to the parent type class, e.g. `CommutativeSemigroup` and 
+`CommutativeArrow`.
+
 #### TODO:
+
 Once we drop 2.10 support, AnyVal-extending class constructor parameters can be marked as private.