Update README.md

The inclusion of SteppedCheckout::setupSteps() in [mysite]_config.php overrides the Shipping Method of the Checkout Page (actually pushes it to the last entry in the array).  This can possibly occur if a developer has read the Silverstripe Shop's Multi-Step Checkout instructions first, before installing this module.
Merge pull request #17 from AntonyThorpe/patch-3

Update README.md