Add documentation about the endpoints

[Pigamer37]

By reading the code I found out that you can validate with URL parameters, both a public feed (url=...) and raw xml data (rawdata=...), but RSS Board's docs about the tool seem to imply you can only validate the former.

Also, a bit of documentation in the README could be useful, like mentioning those parameters and maybe some other stuff like saying data must be sent via GET, etc.