Merge pull request #18 from x-graphql/docs

docs: add README

README.md

@@ -3,3 +3,137 @@ Schema Gateway
 
 ![unit tests](https://github.com/x-graphql/schema-gateway/actions/workflows/unit_tests.yml/badge.svg)
 [![codecov](https://codecov.io/gh/x-graphql/schema-gateway/graph/badge.svg?token=ODMTWZfOrL)](https://codecov.io/gh/x-graphql/schema-gateway)
+
+
+![Copyright GraphQL Stitching](https://the-guild.dev/graphql/stitching/_next/static/media/distributed-graph.237d74a1.png)
+
+**Image source: [GraphQL Stitching](https://the-guild.dev/graphql/stitching)**
+
+
+Getting started
+---------------
+
+Install this package via [Composer](https://getcomposer.org)
+
+```shell
+composer require x-graphql/schema-gateway
+```
+
+Add `http-schema` package for creating and executing GraphQL schema over HTTP:
+
+```shell
+composer require x-graphql/http-schema
+```
+
+Usages
+------
+
+```php
+<?php
+
+require __DIR__ . '/vendor/autoload.php';
+
+use GraphQL\GraphQL;
+use GraphQL\Type\Definition\ObjectType;
+use GraphQL\Type\Definition\Type;
+use GraphQL\Utils\SchemaPrinter;
+use GraphQL\Type\Schema;
+use XGraphQL\HttpSchema\HttpDelegator;
+use XGraphQL\HttpSchema\HttpSchemaFactory;
+use XGraphQL\SchemaGateway\MandatorySelectionSetProviderInterface;
+use XGraphQL\SchemaGateway\Relation;
+use XGraphQL\SchemaGateway\RelationArgumentResolverInterface;
+use XGraphQL\SchemaGateway\RelationOperation;
+use XGraphQL\SchemaGateway\SchemaGatewayFactory;
+use XGraphQL\SchemaGateway\SubSchema;
+
+$localSchema = new Schema([
+    'query' => new ObjectType([
+        'name' => 'Query',
+        'fields' => [
+            'person' => [
+                'type' => new ObjectType([
+                    'name' => 'Person',
+                    'fields' => [
+                        'name' => Type::nonNull(Type::string()),
+                        'fromCountry' => Type::nonNull(Type::string()),
+                    ],
+
+                ]),
+                'resolve' => fn() => ['name' => 'John Doe', 'fromCountry' => 'VN']
+            ],
+        ],
+    ]),
+]);
+$localSubSchema = new SubSchema('local', $localSchema);
+
+$remoteSchema = HttpSchemaFactory::createFromIntrospectionQuery(
+    new HttpDelegator('https://countries.trevorblades.com/'),
+);
+$remoteSubSchema = new SubSchema('remote', $remoteSchema);
+
+$countryRelation = new Relation(
+    'Person',
+    'remoteCountry',
+    RelationOperation::QUERY,
+    'country',
+    new class implements RelationArgumentResolverInterface, MandatorySelectionSetProviderInterface {
+        public function shouldKeep(string $argumentName, Relation $relation): bool
+        {
+            return false;
+        }
+
+        public function resolve(array $objectValue, array $currentArgs, Relation $relation): array
+        {
+            return ['code' => $objectValue['fromCountry']];
+        }
+
+        public function getMandatorySelectionSet(Relation $relation): string
+        {
+            return '{ fromCountry }';
+        }
+    }
+);
+
+$schemaGateway = SchemaGatewayFactory::create([$localSubSchema, $remoteSubSchema], [$countryRelation]);
+
+$query = <<<'GQL'
+query {
+  continents {
+    name
+  }
+
+  person {
+    name
+    remoteCountry {
+      name
+      code
+    }
+  }
+}
+GQL;
+
+var_dump(SchemaPrinter::doPrint($schemaGateway));
+var_dump(GraphQL::executeQuery($schemaGateway, $query)->toArray());
+``` 
+
+
+**Rules when merging sub schemas:**
+
++ Top-level field names need to be unique across all merged schemas (case-sensitive match).
++ Types with the exact same name and structure will be merged. But types with the same name but different structure will result in type conflicts.
+
+Inspiration
+-----------
+
+This library has been inspired by many others related work including:
+
++ [Hasura](https://hasura.io)
++ [GraphQL Stitching](https://the-guild.dev/graphql/stitching/)
+
+Thanks to all the great people who created these projects!
+
+Credits
+-------
+
+Created by [Minh Vuong](https://github.com/vuongxuongminh)