docs: update README

Author: mkurapov

README.md

@@ -21,7 +21,11 @@ The Open Payments APIs are a collection of three sub-systems:
 - A **resource server** which exposes APIs for performing functions against the underlying accounts
 - A **authorisation server** which exposes APIs compliant with the [GNAP](https://datatracker.ietf.org/doc/html/draft-ietf-gnap-core-protocol) standard for getting grants to access the resource server APIs
 
-This repository hosts the documentation for the Open Payments APIs, which is published on [openpayments.dev](https://openpayments.dev)
+The three Open Payments OpenAPI specifications are found in the [open-payments-specifications](https://github.com/interledger/open-payments-specifications) repository, while this repository hosts the documentation for the APIs, published on [openpayments.dev](https://openpayments.dev). 
+
+### Open Payments SDKs
+
+The Open Payments SDKs provide developers with pre-built functions that simplify interactions with the Open Payments API. Currently, we offer a [TypeScript/NodeJS SDK](https://github.com/interledger/open-payments-node/tree/main/packages/open-payments) and a [PHP SDK](https://github.com/interledger/open-payments-php), with plans to expand to additional languages and frameworks in the future.
 
 ### New to Interledger?