Add validation rule that operation types exist (#955)

* Add validation rule that operation types exist

Right now the spec says that, for example, if the schema does not define
a mutation root type, then the schema does not support mutations.  But
there's no validation rule for it, which means that many parsers
(including graphql-js) treat a mutation as valid against such a schema.
(Indeed, many end up considering *any* mutation as valid, since they
don't know what type to validate the root selection set against.)  This
commit adds a validation rule to make the schema text explicit.

Slated for discussion at the June 2 working group meeting.  See also
graphql/graphql-js#3592.

* prettier

* typofix

Co-authored-by: Benjie Gillam <[email protected]>

* indentation

Co-authored-by: Benjie Gillam <[email protected]>

* Add 5.2.4 Operation Type Configuration

* Update spec/Section 5 -- Validation.md

* Update spec/Section 5 -- Validation.md

* Update spec/Section 5 -- Validation.md

* Apply suggestions from code review

Co-authored-by: Benjie <[email protected]>

* Reformat

* Undo changes to Section 3

* Editorial

* Move Shane's content

* More editorial

* More editorial

* Update spec/Section 5 -- Validation.md

Co-authored-by: Shane Krueger <[email protected]>

* Update spec/Section 5 -- Validation.md

---------

Co-authored-by: Benjie Gillam <[email protected]>
Co-authored-by: Shane Krueger <[email protected]>

Author: 3 people

spec/Section 5 -- Validation.md

@@ -135,6 +135,51 @@ extend type Dog {
 
 ## Operations
 
+### All Operation Definitions
+
+#### Operation Type Existence
+
+**Formal Specification**
+
+- For each operation definition {operation} in the document:
+  - Let {rootOperationType} be the _root operation type_ in {schema}
+    corresponding to the kind of {operation}.
+  - {rootOperationType} must exist.
+
+**Explanatory Text**
+
+A schema defines the _root operation type_ for each kind of operation that it
+supports. Every schema must support `query` operations, however support for
+`mutation` and `subscription` operations is optional.
+
+If the schema does not include the necessary _root operation type_ for the kind
+of an operation defined in the document, that operation is invalid since it
+cannot be executed.
+
+For example given the following schema:
+
+```graphql example
+type Query {
+  hello: String
+}
+```
+
+The following operation is valid:
+
+```graphql example
+query helloQuery {
+  hello
+}
+```
+
+While the following operation is invalid:
+
+```graphql counter-example
+mutation goodbyeMutation {
+  goodbye
+}
+```
+
 ### Named Operation Definitions
 
 #### Operation Name Uniqueness