clarify immutability requirements

subjects/pick-omit/README.md

@@ -3,10 +3,12 @@
 ### Instructions
 
 Create a `pick` function that takes an object and keys.
-This function will pick only the keys, in the second argument, from the object passed as argument.
+This function create a new object that pick only the keys, in the second argument, from the object passed as argument.
 
 Create a `omit` function that takes an object and keys.
-This function will omit only the keys, in the second argument, from the object passed as argument.
+This function create a new object that omit only the keys, in the second argument, from the object passed as argument.
+
+> Those functions are pure and must not modify the given object
 
 ### Notions