Add description to Schema

Author: IvanGoncharov

spec/Appendix B -- Grammar Summary.md

@@ -194,7 +194,7 @@ TypeSystemExtension :
   - SchemaExtension
   - TypeExtension
 
-SchemaDefinition : schema Directives[Const]? { OperationTypeDefinition+ }
+SchemaDefinition : Description? schema Directives[Const]? { OperationTypeDefinition+ }
 
 SchemaExtension :
   - extend schema Directives[Const]? { OperationTypeDefinition+ }

spec/Section 3 -- Type System.md

@@ -40,7 +40,7 @@ GraphQL service which is itself an extension of another GraphQL service.
 
 ## Schema
 
-SchemaDefinition : schema Directives[Const]? { RootOperationTypeDefinition+ }
+SchemaDefinition : Description? schema Directives[Const]? { RootOperationTypeDefinition+ }
 
 RootOperationTypeDefinition : OperationType : NamedType
 
@@ -183,16 +183,23 @@ Markdown syntax (as specified by [CommonMark](http://commonmark.org/)). In the
 type system definition language, these description strings (often {BlockString})
 occur immediately before the definition they describe.
 
-All GraphQL types, fields, arguments and other definitions which can be
-described should provide a {Description} unless they are considered self
-descriptive.
+GraphQL schema and all other definitions (e.g. types, fields, arguments, etc.)
+which can be described should provide a {Description} unless they are considered
+self descriptive.
 
 As an example, this simple GraphQL schema is well described:
 
 ```graphql example
 """
 A simple GraphQL schema which is well described.
 """
+schema {
+  query: Query
+}
+
+"""
+Root type for all your queries
+"""
 type Query {
   """
   Translates a string from a given language into a different language.

spec/Section 4 -- Introspection.md

@@ -117,6 +117,7 @@ The schema of the GraphQL schema introspection system:
 
 ```graphql
 type __Schema {
+  description: String
   types: [__Type!]!
   queryType: __Type!
   mutationType: __Type