README: ESTKme coupon code / link broken

I was about to order from ESTKme and remembered the `MiniLPA` coupon.
So I used it, but it only gives 5% off (instead of 10%) and only on SIM cards. Not on the card reader.

Also the ECOlite is replaced by the AIR. And the provided link does not apply the coupon code automatically, you have to enter it into the cart by hand.

You might want to check that. :)