Add experimental coroutine executor documentations

Author: mcg-web

README.md

@@ -44,6 +44,7 @@ Documentation
   - [GraphQL schema language](docs/definitions/graphql-schema-language.md)
   - [Schema](docs/definitions/schema.md)
   - [Resolver](docs/definitions/resolver.md)
+  - [Experimental coroutine executor](docs/definitions/coroutine-executor.md)
   - [Solving N+1 problem](docs/definitions/solving-n-plus-1-problem.md)
   - [Mutation](docs/definitions/mutation.md)
   - [Relay](docs/definitions/relay/index.md)

docs/definitions/coroutine-executor.md

@@ -0,0 +1,11 @@
+Experimental coroutine executor
+================================
+
+[The experimental coroutine executor](https://github.com/webonyx/graphql-php/blob/0.13.x/UPGRADE.md#try-it-experimental-executor-with-improved-performance)
+can be enabled using configuration:
+
+```yaml
+overblog_graphql:
+    definitions:
+        use_experimental_executor: true
+```

docs/definitions/index.md

@@ -7,6 +7,7 @@
 
 ## Go further
 
+-   [Experimental coroutine executor](coroutine-executor.md)
 -   [Solving N+1 problem](solving-n-plus-1-problem.md)
 -   [Resolver](resolver.md)
 -   [Mutation](mutation.md)