Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -91,6 +91,7 @@ After submitting your pull request, request a review from the project maintainer
 
 # Naming conventions
 
-- function names, `snake_case` is preferred
-- type paramaters, one character starting from `A` is preferred, e.g, `fn map[A,B](self : Array[A], f : (A) -> (B)) -> Array[B]`
-- type names, `CamlCase` is preferred, if one package is centered around one specific type, short name `T` is preferred, e.g, `@sorted_set.T`
+- function names, `snake_case` is preferred.
+- type paramaters, one character starting from `A` is preferred, e.g, `fn map[A,B](self : Array[A], f : (A) -> (B)) -> Array[B]`, for some established
+  conventions, `Map[K,V]` it is also accepted.
+- type names, `CamlCase` is preferred, if one package is centered around one specific type, short name `T` is preferred, e.g, `@sorted_set.T`.