Add missing coercion rule

Author: benjie

spec/Section 3 -- Type System.md

@@ -1595,6 +1595,9 @@ is constructed with the following rules:
   definition does not provide a default value, the input object field
   definition's default value should be used.
 
+* If the input object is a oneof input object, the map must contain exactly one
+  entry, and the value of that entry must not be {null}.
+
 Following are examples of input coercion for an input object type with a
 `String` field `a` and a required (non-null) `Int!` field `b`: