Add 5.2.4 Operation Type Exists

spec/Section 5 -- Validation.md

@@ -332,6 +332,41 @@ contain any number of operations, each of which may contain different root
 fields. When executed, a document containing multiple subscription operations
 must provide the operation name as described in {GetOperation()}.
 
+### Operation Type Exists
+
+#### Formal Specification
+
+For each operation definition in the document:
+
+- Let {operationType} be the type of the operation (`query`, `mutation`, or
+  `subscription`).
+- The corresponding _root operation type_ for {operationType} must be defined in
+  the schema.
+
+#### Explanatory Text
+
+A schema defines the root operation types that it supports. Any document that
+contains an operation of a type unsupported by the schema is invalid, since such
+an operation cannot be executed.
+
+While query operations are required for all schemas, mutation and subscription
+operations are optional. If the schema does not include the necessary type for a
+mutation or subscription operation defined in the document, it will be
+considered invalid.
+
+For example, the following document is valid if the schema includes a Mutation
+type, but invalid if it does not:
+
+```graphql example
+mutation {
+  likeStory(storyID: 12345) {
+    story {
+      likeCount
+    }
+  }
+}
+```
+
 ## Fields
 
 ### Field Selections