Add Readme to the 'revalidate' endpoint

[Alessandro100]

The revalidate route is an endpoint that is used by external systems and would greatly benefit from a readme explaining what it does, and how to use it

Add a new Readme to the folder and the documentation would include

• the possible input types and what they do
• and the format of the body

Also add an extra verification if the user sends a type or they input a type that is not valid, that the endpoint will return a 500 stating 'invalid or missing type parameter'