Added tindie.com to sites in README.
Added tindie.com to sites in README.

Parisson/cartridge,wbtuomela/cartridge,ryneeverett/cartridge,dsanders11/cartridge,wbtuomela/cartridge,syaiful6/cartridge,Kniyl/cartridge,dsanders11/cartridge,traxxas/cartridge,viaregio/cartridge,jaywink/cartridge-reservable,jaywink/cartridge-reservable,stephenmcd/cartridge,ryneeverett/cartridge,wbtuomela/cartridge,Parisson/cartridge,stephenmcd/cartridge,wyzex/cartridge,traxxas/cartridge,syaiful6/cartridge,viaregio/cartridge,ryneeverett/cartridge,stephenmcd/cartridge,syaiful6/cartridge,jaywink/cartridge-reservable,Kniyl/cartridge,wyzex/cartridge,Parisson/cartridge,wyzex/cartridge,dsanders11/cartridge,Kniyl/cartridge,traxxas/cartridge