Add README section for API contract.

Author: G8XSU

README.md

@@ -56,6 +56,10 @@ VSS execution is split into two phases: Phase I prioritizes recovery and single-
 multi-device use. Phase I is ready to use and integrated within [LDK-node]. Phase II will be subject to monitoring for
 demand from wallets and may slip to a later date. Refer to [LDK-Roadmap] for more details.
 
+### API
+
+See the [VSS API contract] for details.
+
 ### Implementation
 
 Currently, VSS-server has a Java-based implementation and is ready to use. Support for a Rust-based VSS-server is a work
@@ -77,7 +81,8 @@ VSS is also integrated with [LDK-node] v0.4.x as alpha support.
 * **Scaling**: VSS itself is stateless and can be horizontally scaled easily. VSS can be configured to point to a
   PostgreSQL cluster, and further scaling considerations need to be addressed in the PostgreSQL cluster.
 * **Using with LDK-node**: [LDK-node] can be easily configured to run with VSS as primary storage. It is integrated in
-  LDK-node (written in Rust) using [VSS-rust-client], and there is also support for other languages such as Swift, Kotlin,
+  LDK-node (written in Rust) using [VSS-rust-client], and there is also support for other languages such as Swift,
+  Kotlin,
   and Python through [UniFFI] bindings.
     ```rust
     use ldk_node::Builder;
@@ -107,6 +112,8 @@ If you encounter any issues or have questions, feel free to open an issue on
 the [GitHub repository](https://github.com/lightningdevkit/vss-server/issues). For further assistance or to discuss the
 development of VSS, you can reach out to us in the [LDK Discord] in the `#vss` channel.
 
+[VSS API contract]: https://github.com/lightningdevkit/vss-server/blob/main/proto/vss.proto
+
 [VSS-rust-client]: https://github.com/lightningdevkit/vss-rust-client
 
 [LDK-node]: https://github.com/lightningdevkit/ldk-node