Incorrect information in the Adding Custom Policies doc

There are some issues with the documentation describing how to add custom policies:
https://access.redhat.com/documentation/en-us/red_hat_3scale/2.2/html/deployment_options/apicast_policies#adding_custom_policies_to_apicast

1. AMP_RELEASE should be 2.2.0, as this is the value used in the amp.yml template for the image stream

2. Some steps of the process are missing:

• push the changes to the fork of the GitHub repo (it doesn't take the changes from the local clone)
• update the GitHub URL to the one with the custom policy in openshift.yml template

The steps need to be verified and fixed.