add creation of category and subcategory into readme and add test for…

[alirezat66]

Custom Categories Are Already Supported

Step 1 — Register it on the RoutingBuilder:

FlexTrack.configure(
  RoutingBuilder()
    .defineCategory('experiments', description: 'A/B test and feature flag events')
    .defineCategory('payments', description: 'Payment flow events')
    // ... rest of your config
);