add @experimental

spec/Section 3 -- Type System.md

@@ -879,6 +879,8 @@ of rules must be adhered to by every Object type in a GraphQL schema.
          {"\_\_"} (two underscores).
       2. The argument must accept a type where {IsInputType(argumentType)}
          returns {true}.
+      3. If argument type is Non-Null and a default value is not defined:
+         - The `@experimental` directive shall not be applied to this argument.
 3. An object type may declare that it implements one or more unique interfaces.
 4. An object type must be a super-set of all interfaces it implements:
    1. Let this object type be {objectType}.
@@ -995,6 +997,22 @@ type ExampleType {
 }
 ```
 
+### Experimental fields
+
+Fields in an object may be marked as experimental as deemed necessary by the
+application. Experimental fields can be included in selection sets but might be
+changed in a backward incompatible manner. The fields should be appropriately
+treated in documentation and tooling.
+
+When using the type system definition language, `@experimental` directives are
+used to indicate that a field is experimental:
+
+```graphql example
+type ExampleType {
+  newField: String @experimental
+}
+```
+
 ### Object Extensions
 
 ObjectTypeExtension :
@@ -1652,6 +1670,8 @@ input ExampleInputObject {
       {"\_\_"} (two underscores).
    3. The input field must accept a type where {IsInputType(inputFieldType)}
       returns {true}.
+   4. If input field type is Non-Null and a default value is not defined:
+      - The `@experimental` directive shall not be applied to this input field.
 3. If an Input Object references itself either directly or through referenced
    Input Objects, at least one of the fields in the chain of references must be
    either a nullable or a List type.
@@ -1895,6 +1915,10 @@ GraphQL implementations that support the type system definition language must
 provide the `@deprecated` directive if representing deprecated portions of the
 schema.
 
+GraphQL implementations that support the type system definition language must
+provide the `@experimental` directive if representing experimental portions of
+the schema.
+
 GraphQL implementations that support the type system definition language should
 provide the `@specifiedBy` directive if representing custom scalar definitions.
 
@@ -2067,6 +2091,40 @@ type ExampleType {
 }
 ```
 
+### @experimental
+
+```graphql
+directive @experimental(
+  reason: String = "Experimental"
+) on FIELD_DEFINITION | ARGUMENT_DEFINITION | INPUT_FIELD_DEFINITION | ENUM_VALUE
+```
+
+The `@experimental` _built-in directive_ is used within the type system
+definition language to indicate experimental portions of a GraphQL service's
+schema, such as experimental fields on a type, arguments on a field, input
+fields on an input type, or values of an enum type.
+
+In this example type definition, `newField` and `newArg` are experimental and
+might be changed in a backward incompatible way. For an example, it could be
+renamed and/or its type may be changed.
+
+```graphql example
+type ExampleType {
+  newField: String @experimental
+
+  anotherField(newArg: String @experimental): String
+}
+```
+
+The `@experimental` directive must not appear on required (non-null without a
+default) arguments or input object field definitions.
+
+```graphql counter-example
+type ExampleType {
+  invalidField(newArg: String! @experimental): String
+}
+```
+
 ### @specifiedBy
 
 ```graphql