Add Composer requirement to PHP platform guide

Because it seems that Composer is required in the steps below

Author: brendanpetty

docs/platforms/php/index.mdx

@@ -12,6 +12,7 @@ categories:
 
 * You need a [Sentry account](https://sentry.io/signup/) and project
 * Your application needs to run on PHP 7.2 or later
+* You need to be using Composer
 * Read one of these other guides if you use <PlatformLink to="/guides/laravel">Laravel</PlatformLink> or <PlatformLink to="/guides/symfony">Symfony</PlatformLink>
 
 ## Features