Added note about testing credit card numbers.

I, like many others, are curious about this project but paranoid about typing their credit card number into fields.

I've added a link in the notes section that has credit card numbers specifically for testing.

It might also be good to place one of these test #'s into the placeholder so idiots like me don't type the placeholder and wonder why this project is so neat.
Merge pull request #9 from patrick-mcdougle/patch-1

Added note about testing credit card numbers.