add parameter list (#128)
add parameter list (#128)

* add parameter list

Having a parameter list in the README removes the need to dig into
the code in order to determine what parameters are available on
the parameterized class.
ocadotechnology/puppetlabs-dhcp,archii/puppetlabs-dhcp,puppetlabs/puppetlabs-dhcp,ocadotechnology/puppetlabs-dhcp,archii/puppetlabs-dhcp,puppetlabs/puppetlabs-dhcp