smallrye#521: add documentation

docs/federation.md

@@ -0,0 +1,41 @@
+# Federation
+
+To enable support from [GraphQL Federation](https://www.apollographql.com/docs/federation), simply set the `smallrye.graphql.federation.enabled` config key to `true`.
+
+You can add the Federation directives by using the equivalent Java annotation, e.g. to extend a `Product` entity with a `price` field, you can write a class:
+
+```java
+package org.example.price;
+
+import org.eclipse.microprofile.graphql.Id;
+
+import io.smallrye.graphql.api.federation.Extends;
+import io.smallrye.graphql.api.federation.Key;
+
+@Extends @Key(fields = "id")
+public class Product {
+    @Id
+    private String id;
+
+    @Description("The price in cent")
+    private Integer price;
+
+    // getters and setters omitted
+}
+```
+
+The GraphQL Schema then contains:
+
+```graphql
+type Product @extends @key(fields : ["id"]) {
+  id: ID
+  price: Int
+}
+
+union _Entity = Product
+
+type Query {
+  _entities(representations: [_Any!]!): [_Entity]!
+  _service: _Service!
+}
+```

mkdocs.yml

@@ -7,6 +7,7 @@ nav:
   - Server side features:
       - Customizing JSON deserializers: 'custom-json-deserializers.md'
       - Directives: 'directives.md'
+      - Federation: 'federation.md'
       - Custom error extensions: 'custom-error-extensions.md'
   - Typesafe client:
       - Basic usage: 'typesafe-client-usage.md'