Suggest symfony/yaml, and say so when a YAML specification needs it

README.md

@@ -32,6 +32,9 @@ composer require radebatz/openapi-verifier
 ```
 After that all required classes should be available in your project.
 
+Some features need further packages, such as a framework adapter or YAML support. They are listed
+as suggestions, and `composer suggests radebatz/openapi-verifier` shows them.
+
 ## Usage
 ### The verifier
 `OpenApiVerifier` checks a response against the specification. It needs no test case and no framework,
@@ -47,6 +50,8 @@ $verifier = OpenApiVerifier::fromSpecification(__DIR__ . '/specifications/users.
 $verifier->verify($request, $response);
 ```
 
+The specification can be a JSON or YAML file, or an already decoded object.
+
 How the response is looked up:
 * **Path**: a literal match first, then the path templates, so `/user/1` finds `/user/{id}`. When several
   templates match, the one with the most literal characters wins. A base path from `servers` is stripped

composer.json

@@ -90,6 +90,7 @@
         "guzzlehttp/guzzle": "To verify responses in a Guzzle client, with the Guzzle middleware",
         "laravel/framework": "To use the Laravel adapter",
         "slim/slim": "To use the Slim adapter",
+        "symfony/yaml": "To load YAML specifications",
         "zircote/swagger-php": "To generate the specification from PHP attributes when none is given"
     }
 }

src/OpenApiSpecificationLoader.php

@@ -22,6 +22,10 @@ protected function resolveSpecification(mixed $specification): \stdClass
         $resolved = null;
         if (is_string($specification)) {
             if (preg_match('/\.ya?ml$/i', $specification)) {
+                if (!class_exists(Yaml::class)) {
+                    throw new \LogicException(sprintf('Unable to load specification "%s" as symfony/yaml is not installed. Try running "composer require symfony/yaml".', $specification));
+                }
+
                 try {
                     $resolved = (object) Yaml::parseFile($specification, Yaml::PARSE_OBJECT_FOR_MAP);
                 } catch (ParseException $parseException) {