Add Lookup::Test.reset to README
Add Lookup::Test.reset to README

The method `#reset` for clearing stubs
was previously unmentioned in the README

brian-ewell/geocoder,TrangPham/geocoder,Devetzis/geocoder,tiramizoo/geocoder,donbobka/geocoder,aceunreal/geocoder,rdlugosz/geocoder,ankane/geocoder,alexreisner/geocoder