changelog: explicitly link to graphql-php release

which documents the breaking changes in detail

Author: mfn

CHANGELOG.md

@@ -13,6 +13,7 @@ CHANGELOG
 ---------------------
 ## Breaking changes
 - Upgrade to webonyx/graphql-php 14.0.0 [\#645 / mfn](https://github.com/rebing/graphql-laravel/pull/645)
+  Be sure to read up on breaking changes in graphql-php => https://github.com/webonyx/graphql-php/releases/tag/v14.0.0
 - Remove support for Laravel < 6.0 [\#651 / mfn](https://github.com/rebing/graphql-laravel/pull/651)
   This also bumps the minimum required version to PHP 7.2