Added Grav to integrations (php-debugbar#292)

I've added Grav to the list of integrations as we've been using DebugBar in Grav for the past couple of years.  Thanks for the great project!

README.md

@@ -41,6 +41,7 @@ Integrations with other frameworks:
   - [Zend Framework 2](https://github.com/snapshotpl/ZfSnapPhpDebugBar)
   - [Phalcon](https://github.com/snowair/phalcon-debugbar)
   - [SilverStripe](https://github.com/lekoala/silverstripe-debugbar)
+  - [Grav CMS](https://getgrav.org)
   - Framework-agnostic middleware and PSR-7 with [php-middleware/phpdebugbar](https://github.com/php-middleware/phpdebugbar).
 
 *(drop me a message or submit a PR to add your DebugBar related project here)*