feat(demo): add simple demo script

Author: bighappyface

demo/README.md

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+## JsonSchema\Validator Demo
+
+This demo script uses the example from the root README.md and provides sample JSON files for testing `JsonSchema\Validator`.
+
+To change or replace the JSON schema document, please edit `/path/to/json-schema/demo/schema.json`.
+
+To change or replace the JSON document that is validated, please edit `/path/to/json-schema/demo/data.json`.
+
+To run the demo, change the path in the following example and run it in your terminal.
+
+```
+cd /path/to/json-schema/demo
+php demo.php // The supplied JSON validates against the schema.
+```

demo/data.json

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+{
+  "foo":"bar"
+}

demo/demo.php

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+<?php
+require(__DIR__ . '/../vendor/autoload.php');
+
+$data = json_decode(file_get_contents('data.json'));
+
+// Validate
+$validator = new JsonSchema\Validator;
+$validator->check($data, (object)['$ref' => 'file://' . realpath('schema.json')]);
+
+if ($validator->isValid()) {
+    echo "The supplied JSON validates against the schema.\n";
+} else {
+    echo "JSON does not validate. Violations:\n";
+    foreach ($validator->getErrors() as $error) {
+        echo sprintf("[%s] %s\n", $error['property'], $error['message']);
+    }
+}

demo/schema.json

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+{
+  "type": "object"
+}