Add definition for Card Transaction Type

This is proposed in conjunction with discussion here thephpleague/omnipay-common#160 and in support of the proposed change here
eileenmcnaughton/omnipay-eway@ad2b21a

README.md

@@ -349,6 +349,7 @@ All gateways will accept a subset of these options:
 * clientIp
 * returnUrl
 * cancelUrl
+* cardTransactionType (one of 'ecommerce', 'continuous', 'moto' or 'retail', see below for more)
 
 Pass the options through to the method like so:
 
@@ -492,6 +493,18 @@ recurring billing profiles. Also in most cases token billing will cover your nee
 store a credit card then charge it on whatever schedule you like. Feel free to get in touch if
 you really think this should be a core feature and worth the effort.
 
+## Card transaction Type
+
+Some gateways support multiple card transaction types with different requirements. For example eWay
+requires the cvn to be re-entered in an 'ecommerce' token transaction where the card owner is present
+but not when processing a 'continuous' transaction - such as a monthly donation.
+
+You can set this as:
+- ecommerce: card owner present online; 3D-Secure supported; saved tokens may be used.
+- continuous: card owner not present; repeats a previous approved transaction.
+- moto: card owner MAY be present; NO 3D-Secure; saved tokens may be used.
+- retail: POS machine or similar. Card owner present, PIN authorization possible. No 3D-Secure, no saved tokens.
+
 ## Incoming Notifications
 
 Some gateways (e.g. Cybersource, GoPay) offer HTTP notifications to inform the merchant about the completion (or, in