Add req:read_cached() to readme

[iDneprov]

I needed to extract the body of the query as a string without parsing json.
I expected req:request_line() to return the query body because of it's description "returns the request body". But in my case it returned "GET /tokens/1? HTTP/1.1".
Please, change description of req:request_line() and add req:read_cached() to readme.