Added hello world example

examples/00-hello-world/README.md

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+# Hello world
+Clean and simple single-file example of main GraphQL concepts originally proposed and 
+implemented by [Leo Cavalcante](https://github.com/leocavalcante)
+
+### Run locally
+```
+php -S localhost:8080 ./graphql.php
+```
+
+### Try query
+```
+curl http://localhost:8080 -d "query { echo(message: \"Hello World\") }"
+```
+
+### Try mutation
+```
+curl http://localhost:8080 -d "mutation { sum(x: 2, y: 2) }"
+```

examples/00-hello-world/graphql.php

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+<?php
+// Test this using following command
+// php -S localhost:8080 ./graphql.php
+require_once '../../vendor/autoload.php';
+
+use GraphQL\Type\Definition\ObjectType;
+use GraphQL\Type\Definition\Type;
+use GraphQL\Schema;
+use GraphQL\GraphQL;
+
+try {
+    $queryType = new ObjectType([
+        'name' => 'Query',
+        'fields' => [
+            'echo' => [
+                'type' => Type::string(),
+                'args' => [
+                    'message' => ['type' => Type::string()],
+                ],
+                'resolve' => function ($root, $args) {
+                    return $root['prefix'].$args['message'];
+                }
+            ],
+        ],
+    ]);
+
+    $mutationType = new ObjectType([
+        'name' => 'Calc',
+        'fields' => [
+            'sum' => [
+                'type' => Type::int(),
+                'args' => [
+                    'x' => ['type' => Type::int()],
+                    'y' => ['type' => Type::int()],
+                ],
+                'resolve' => function ($root, $args) {
+                    return $args['x'] + $args['y'];
+                },
+            ],
+        ],
+    ]);
+
+    $schema = new Schema([
+        'query' => $queryType,
+        'mutation' => $mutationType,
+    ]);
+
+    $rawInput = file_get_contents('php://input');
+
+    $rootValue = ['prefix' => 'You said: '];
+    $result = GraphQL::execute($schema, $rawInput, $rootValue);
+} catch (\Exception $e) {
+    $result = [
+        'error' => [
+            'message' => $e->getMessage()
+        ]
+    ];
+}
+header('Content-Type: application/json; charset=UTF-8');
+echo json_encode($result);
+