Add docs for newly added (since 0.2.11) HashMap.!?.

Author: m-renaud

docs/hash-map.rst

@@ -229,8 +229,13 @@ Lookup an entry in the map (lookup)
     HashMap.lookup :: k -> HashMap k v -> Maybe v
     HashMap.lookup key m = ...
 
+    HashMap.!? :: HashMap k v -> k -> Maybe v
+    HashMap.!? m key = ...
+
 :haddock_short:`/Data.HashMap.Strict#lookup` the value corresponding to the given
-``key``, returns ``Nothing`` if the key is not present.
+``key``, returns ``Nothing`` if the key is not present. The ``!?`` operator
+(*since 0.2.11*) is a flipped version of ``lookup`` and can often be imported
+unqualified.
 
 If you want to provide a default value if the key doesn't exist you can use:
 
@@ -247,6 +252,9 @@ For example::
     HashMap.lookup 1 (HashMap.fromList [(1,"one"),(2,"two"),(3,"three")])
     > Just "one"
 
+    (HashMap.fromList [(1,"one"),(2,"two"),(3,"three")]) !? 1
+    > Just "one"
+
     HashMap.lookupDefault "?" k HashMap.empty
     > "?"