Update README.md

Paypal now requires 'tax_amount' to be set. Also the totals were wrong and were throwing an error. The way they are added up is counter-intuitive so I added comments to clarify.
Edited intro text so it reads easier

It was already fine but just added a missing word and made a couple edits so the summary text reads easier. Slight improvement for the troubled mind of any programmer who came here after mucking through paypal's docs for the past 2 hours.
Merge pull request #19 from carlbeckel/master

Update README.md to prevent error response