Add List.contains to util

src/Util.md

@@ -31,6 +31,26 @@ Applies a function to the contents of a `Left` if the value of the given
   mapLeft f (Right x) = Right x
 ```
 
+## List
+
+<!-- idris 
+namespace List
+-->
+
+### contains
+
+Returns `True` if the list contains the given value
+
+```idris
+export
+contains : Eq a => a -> List a -> Bool
+contains x [] = False
+contains x (y :: xs) = 
+  if x == y 
+    then True
+    else contains x xs
+```
+
 ## Vectors
 
 Define some operations for pairs of numbers, treating them roughly like vectors