Update README testing instructions

[overint]

As per #134 it seems that calls from the IPN simulator can no longer be validated, however calls from sandbox accounts work correctly.

This PR updates the documentation to reflect that.

[overint]

Will merge this in two days if there are no comments @braebot @randstraw

[0x46]

I was able to verify calls from the IPN simulator with the PHP example: gist

this is the verified response from the default IPN simulator settings (Cart Checkout):

paypal_ipn_status = VERIFIED
paypal_ipn_date = 2018-11-20 14:50:28 CET
payment_type = instant
payment_date = 14:50:19 Nov 19, 2018 PST
payment_status = Completed
payer_status = verified
first_name = John
last_name = Smith
payer_email = [email protected]
payer_id = TESTBUYERID01
address_name = John Smith
address_country = United States
address_country_code = US
address_zip = 95131
address_state = CA
address_city = San Jose
address_street = 123 any street
business = [email protected]
receiver_email = [email protected]
receiver_id = [email protected]
residence_country = US
item_name1 = something
item_number1 = AK-1234
quantity = 1
shipping = 3.04
tax = 2.02
mc_currency = USD
mc_fee = 0.44
mc_gross = 12.34
mc_gross_1 = 12.34
mc_handling = 2.06
mc_handling1 = 1.67
mc_shipping = 3.02
mc_shipping1 = 1.02
txn_type = cart
txn_id = 899327589
notify_version = 2.4
custom = xyz123
invoice = abc1234
test_ipn = 1
verify_sign = AnFzcuuaGxJq8NFjw0ci-L2eIk27A-CUVhAal9TSj88b3gWuA8UdkNC3