add authorization to readme

Author: abuzuhri

README.md

@@ -56,6 +56,7 @@ Install-Package CSharpAmazonSpAPI
 - [ ] [Services](https://github.com/amzn/selling-partner-api-docs/blob/main/references/services-api/services.md)
 - [x] [Solicitations](https://github.com/amzn/selling-partner-api-docs/blob/main/references/solicitations-api/solicitations.md)
 - [x] [Token](https://github.com/amzn/selling-partner-api-docs/blob/main/references/tokens-api/tokens_2021-03-01.md) for [doc PII](https://github.com/amzn/selling-partner-api-docs/blob/main/guides/en-US/use-case-guides/tokens-api-use-case-guide/tokens-API-use-case-guide-2021-03-01.md)
+- [x] [Authorization](https://github.com/amzn/selling-partner-api-docs/blob/main/references/authorization-api/authorization.md)
 
 #### Vendor 
 - [ ] [VendorOrders](https://github.com/amzn/selling-partner-api-docs/blob/main/references/vendor-orders-api/vendorOrders.md)