Update README

Merge remote-tracking branch 'origin/master'

Merge pull request bigcommerce#222 from kevinmatter/add-get-order-cou…

…pons

add getOrderCoupons

Merge pull request bigcommerce#217 from bookernath/bump-min-php-56

Bump min PHP to 7.0, start running tests on 7.1 and 7.2

Merge pull request bigcommerce#208 from BVMiko/master

Support newer versions of firebase/php-jwt

Merge pull request bigcommerce#179 from bc-joshroe/api-fix-1

BIG-16297 - Adding some methods to the API Client

Merge pull request bigcommerce#174 from bc-ruth/OMNI-1167

Revert bigcommerce#155

Merge pull request bigcommerce#173 from bc-ruth/OMNI-1259

OMNI-1259 Expose product image resource to Client API

Merge pull request bigcommerce#170 from bc-ruth/OMNI-1024

OMNI-1024 Expose currencies v2 resource to Client API

Merge pull request bigcommerce#163 from bc-bfenton/API-94

API-94 - adding else case to switch back to JSON mode if useXml is false