Documented the subradius property

[jlcd]

I almost reinvented the wheel creating a new lib just to have the option to broaden the search above the 60 places limit, as I didn't know of this property.

A Google search led me to https://github.com/joshtronic/php-googleplaces/issues/5 and thankfuly I didn't need to implement a whole new thing. So I thought this property should be stated in the README.