Update README.md

Added note about cartridge commit requirement
Fixing duplicate orders issue
Fixing or statement
Merge branch 'master' of https://github.com/explodes/cartridge-payments

Conflicts:
	payments/multipayments/forms/base.py
Update base.py

EXISTS is cheaper than COUNT
fixes total multiplication error at paypal and google
Merge pull request #5 from luxnovalabs/develop

Looks good merging in