Add securionpay-php documentation

[laukstein]

README.md has only a single example and https://securionpay.com/docs pages about 3 more examples on how to use securionpay-php. None of them explains securionpay-php parameters and all functions, and responses.
Please list all capabilities of securionpay-php. Perhaps https://securionpay.com/docs/api nex to CURL should contain example of securionpay-php ...