[RFC] Add note recommending directive names to be namespaced (#657)

* Add note recommending directive names to be namespaced

* move to type system section

* Expand note and include example about current rfcs

* Add link to custom directives sub-section from language section

Co-authored-by: Lee Byron <lee@leebyron.com>

spec/Section 2 -- Language.md

@@ -1093,7 +1093,9 @@ Directives can be used to describe additional information for types, fields, fra
 and operations.
 
 As future versions of GraphQL adopt new configurable execution capabilities,
-they may be exposed via directives.
+they may be exposed via directives. GraphQL services and tools may also provide
+additional [custom directives](#sec-Type-System.Directives.Custom-Directives)
+beyond those described here.
 
 **Directive order is significant**
 

spec/Section 3 -- Type System.md

@@ -1650,6 +1650,21 @@ GraphQL implementations that support the type system definition language must
 provide the `@deprecated` directive if representing deprecated portions of
 the schema.
 
+**Custom Directives**
+
+GraphQL services and client tooling may provide additional directives beyond
+those defined in this document. Directives are the preferred way to extend
+GraphQL with custom or experimental behavior.
+
+Note: When defining a directive, it is recommended to prefix the directive's
+name to make its scope of usage clear and to prevent a collision with directives
+which may be specified by future versions of this document (which will not
+include `_` in their name). For example, a custom directive used by Facebook's
+GraphQL service should be named `@fb_auth` instead of `@auth`. This is
+especially recommended for proposed additions to this specification which can
+change during the [RFC process](https://github.com/graphql/graphql-spec/blob/master/CONTRIBUTING.md).
+For example an work in progress version of `@live` should be named `@rfc_live`.
+
 Directives must only be used in the locations they are declared to belong in.
 In this example, a directive is defined which can be used to annotate a field: