Update readme

* Revert HTTP config host precedence order change due to issue with s…

…ome configs.

* Use `PAYPAL_DEBUG` instead of `NODE_ENV` to print debug messages [#289

](#289).

* Handler whitespaces only for response body [#286](#286).
* Validate `mode` to be either `sandbox` or `live` [#269](#269).
* HTTP config host precedence order set as other configs [#231](#231).
* Remove unnecessary setting of PayPal-Request-Id.
* Remove deprecated samples, and other minor bug fixes.

* Update credit card vault URL to /v1/vault/credit-cards/ [#222](#222).

* Fix access token caching when using refresh token config [#226](#226).

* Updated Webhooks Verify event.

* Invoicing API updates [#204](#204).
* Minor bug fixes.

* Added new Third Party Invoicing sample [#157](#157)

* Use default headders for OAuth calls [#155](#155)
* Modify default start_date for Billing Agrement create sample [#138](#138)
* Fix a typo in inline documentation [#135](#138)