docs: link migration guide from README

[#187199940](https://www.pivotaltracker.com/story/show/187199940)

README.md

@@ -2,7 +2,7 @@
 [![test](https://github.com/cloudfoundry/cloud-service-broker/workflows/run-tests/badge.svg?branch=main)](https://github.com/cloudfoundry/cloud-service-broker/actions?query=workflow%3Arun-tests+branch%3Amain)
 [![Go Report Card](https://goreportcard.com/badge/github.com/cloudfoundry/cloud-service-broker)](https://goreportcard.com/report/github.com/cloudfoundry/cloud-service-broker)
 
-**Warning**: ***From Version 1.0.0 onwards the Cloud Service Broker only supports OpenTofu. Custom brokerpaks need to specify an OpenTofu version and the [upgrade process](https://github.com/cloudfoundry/cloud-service-broker/blob/main/docs/terraform-upgrades.md) must be followed for existing instances. Only upgrades from terraform versions 1.5.x are supported ***
+**Warning**: From Version 1.0.0 onwards the Cloud Service Broker only supports OpenTofu. Custom brokerpaks need to specify an OpenTofu version and the [upgrade process](https://github.com/cloudfoundry/cloud-service-broker/blob/main/docs/terraform-upgrades.md) must be followed for existing instances. Only upgrades from terraform versions 1.5.x are supported. For more information, see the [OpenTofu migration guide](./docs/opentofu_migration_guide.md)
 
 # Cloud Service Broker
 An OSBAPI-compliant service broker that uses OpenTofu to create service instances.