Adding text for Payment Processing section

* Uses Saivann's double spend section with a new subhead and a new
  introductory paragraph

* Images were uploaded previously.

Please comment on this pull request even if it's been closed. I
(@harding) will address every single comment received.

Following our normal workflow, I recommend @saivann merge this pull
request in as soon as he's ready to re-run Jekyll on the demo site.