Add warning about space-usage on the Fixed instance

--HG--
extra : rebase_source : 625058aecd476ec8f0140965e04787974ad21b70
extra : histedit_source : 13c9b34b80a918675cffa1311881b00b9e3511d5

Data/Aeson/Types/Instances.hs

@@ -206,6 +206,10 @@ instance HasResolution a => ToJSON (Fixed a) where
     toJSON = Number . realToFrac
     {-# INLINE toJSON #-}
 
+-- | /WARNING:/ Only parse fixed-precision numbers from trusted input
+-- since an attacker could easily fill up the memory of the target
+-- system by specifying a scientific number with a big exponent like
+-- @1e1000000000@.
 instance HasResolution a => FromJSON (Fixed a) where
     parseJSON = withScientific "Fixed" $ pure . realToFrac
     {-# INLINE parseJSON #-}