add note about object1 being renamed into map

Author: tim-bezhashvyly

upgrade-docs/0.18.md

@@ -108,7 +108,7 @@ This also means that `andThen` and `onError` group together much better than in
 A couple functions have been removed or renamed.
 
   - [`Json.Decode`](http://package.elm-lang.org/packages/elm-lang/core/5.0.0/Json-Decode)
-    - `objectN` becomes `mapN`
+    - `objectN` becomes `mapN` (Note: `object1` becomes `map`) 
     - `tupleN` becomes `mapN` with `index`
     - `(:=)` becomes `field`
     - `andThen` args flip