Merge pull request #40 from cvigorsICBF/2.2.x

Add Body Parsing Middleware reference

Author: weierophinney

docs/book/v1/authorization-server.md

@@ -20,6 +20,22 @@ use Mezzio\Authentication\OAuth2;
 
 $app->post('/oauth2/token', OAuth2\TokenEndpointHandler::class);
 ```
+## Parsing JSON payloads in the token endpoint
+To enable the token endpoint to handle the POST requests in JSON, the [Body Parsing Middleware](https://docs.mezzio.dev/mezzio/v3/features/helpers/body-parse/) helper must be included in the application. 
+
+For example:
+
+```php
+use Mezzio\Authentication\OAuth2;
+use Mezzio\Helper\BodyParams\BodyParamsMiddleware;
+
+$app->post('/oauth2/token', [
+    BodyParamsMiddleware::class,
+    OAuth2\TokenEndpointHandler::class
+], 'auth.token');
+```
+WARNING: Do not pipe the body parsing middleware as generic middleware.
+This ensures that the content body is only parsed when it is actually expected.
 
 ## Add the authorization endpoint