Bit' more self explanation on README

[exocode]

Hi there, at first

THANK YOU! for this great gem :-)

Can you implement to the readme a better explanation on the EXPRESS_CHECKOUT and CALL method?

What's the difference?

many thanks

[aliciatang]

ec is a wrapper on top of call. it prepares the data to be used the static method 'CALL'.
So ec is an instance of a potential transaction, it holds the info(eg token) to be used in call.