Added list of third party modules (mostly payment backends) to README.
Added list of third party modules (mostly payment backends) to README.

wbtuomela/cartridge,ryneeverett/cartridge,dsanders11/cartridge,wyzex/cartridge,traxxas/cartridge,wyzex/cartridge,stephenmcd/cartridge,stephenmcd/cartridge,dsanders11/cartridge,wbtuomela/cartridge,Parisson/cartridge,syaiful6/cartridge,stephenmcd/cartridge,ryneeverett/cartridge,wyzex/cartridge,traxxas/cartridge,Parisson/cartridge,wbtuomela/cartridge,Parisson/cartridge,traxxas/cartridge,syaiful6/cartridge,syaiful6/cartridge,dsanders11/cartridge,ryneeverett/cartridge