docs: add "API Documentation" section to the readme (#413)

Author: xavier-brochard

README.md

@@ -20,6 +20,27 @@ yarn add @metamask/snap-solana-wallet
 
 This dual repository setup allows you to develop and test the Solana Wallet Snap alongside the main MetaMask extension. The snap is installed as a dependency in the MetaMask extension repository, where it can be tested and integrated.
 
+## API Documentation
+
+MetaMask interacts with the Solana Wallet Snap via its [JSON-RPC API](packages/snap/openrpc.json). The complete API specification is documented in the OpenRPC format.
+
+### Viewing the API Documentation
+
+The API documentation is located in [`packages/snap/openrpc.json`](packages/snap/openrpc.json). To view it in a user-friendly format:
+
+1. Go to the [OpenRPC Playground](https://playground.open-rpc.org/), or any other OpenRPC viewer of your liking
+2. Copy the contents of [`packages/snap/openrpc.json`](packages/snap/openrpc.json)
+3. Paste it into the playground's editor
+4. Explore the interactive documentation with method details, parameters, examples, and error specifications
+
+### Available Methods
+
+The API includes methods for:
+
+- **Wallet operations** - Account management, transaction signing and broadcasting, and message signing
+- **Protocol requests** - Direct interaction with Solana RPC methods
+- **Client-only requests** (as defined in [SIP-31](https://github.com/MetaMask/SIPs/blob/main/SIPS/sip-31.md)) - For operations like Swap/Bridge without user confirmation
+
 ## Contributing
 
 We welcome contributions to the Solana Wallet Snap! Please read our [Contributing](docs/contributing.md) guidelines to get started.