Update CONTRIBUTING.md.
Update CONTRIBUTING.md.

Removes references to the old Arvo repository and updates information
about pills.

urbit/urbit,ngzax/urbit,jfranklin9000/urbit,ngzax/urbit,urbit/urbit,jfranklin9000/urbit,ngzax/urbit,jfranklin9000/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit,jfranklin9000/urbit,jfranklin9000/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit