Add rules for how circular references in Input Objects are handled gr…

…aphql#189

spec/Section 3 -- Type System.md

@@ -1360,11 +1360,57 @@ Literal Value            | Variables               | Coerced Value
 
 **Type Validation**
 
-1. An Input Object type must define one or more input fields.
-2. The fields of an Input Object type must have unique names within that
-   Input Object type; no two fields may share the same name.
-3. The return types of each defined field must be an Input type.
+* An Input Object type must define one or more input fields.
+* The fields of an Input Object type must have unique names within that
+  Input Object type; no two fields may share the same name.
+* The return types of each defined field must be an Input type.
 
+Input Objects are generally allowed to reference other Input Objects, even itself.
+If an Input Object references itself either directly or through subordinated
+Input Objects, one of the fields in the chain of references must be either nullable
+or a list. Input Objects with non-nullable circular references are invalid, because there
+is no way to provide a legal value for them.
+
+The following examples are allowed:
+
+```graphql example
+input Example {
+  value: String
+  self: Example
+}
+```
+
+This is fine because `self` may simply omitted at any point in the chain.
+
+```graphql example
+input Example {
+  value: String
+  self: [Example!]!
+}
+```
+
+This also works as `self` can just contain an empty list.
+
+The following examples are invalid:
+
+```graphql counter-example
+input Example {
+  value: String
+  self: Example!
+}
+```
+
+```graphql counter-example
+input First {
+  second: Second!
+  string: String
+}
+
+input Second {
+  second: Second!
+  string: String
+}
+```
 
 ### Input Object Extensions