Add instructions for contributing to Bonobos fork
Update readme with improved process description
Update orders.promo_total in OrderUpdater

NOTE: Cherry-picked from this spree pull request until it gets officially resolved: https://github.com/spree/spree/pull/4607

orders.promo_total field wasn't getting updated in certain cases, like when you add the promo first and then a line item afterward.
the *adjustment* got updated and the correct number was displayed in the cart, but the orders.promo_total value was incorrect.
Fix 500 when updating order email to something inconsistent with the associated user's email

Fixes #4614
temporary hack to ensure correct payment

until we can do the real fix.
real fix will be to have front end pass expected payment amount and then have backend verify that amount.
Pass in order params on order creation so they can be overridden