docs(binding-coap): document PSK usage with CoAPs

packages/binding-coap/README.md

@@ -52,6 +52,78 @@ servient
     });
 ```
 
+## Using PSK with CoAPs (DTLS)
+
+The CoAP binding also supports secure communication over `coaps://` using DTLS with Pre-Shared Keys (PSK).
+
+To use PSK security, define a `psk` security scheme in the Thing Description and provide the credentials when consuming the Thing.
+
+### Thing Description Example (PSK)
+
+```json
+{
+  "title": "SecureThing",
+  "securityDefinitions": {
+    "psk_sc": {
+      "scheme": "psk"
+    }
+  },
+  "security": ["psk_sc"],
+  "properties": {
+    "count": {
+      "type": "integer",
+      "forms": [
+        {
+          "href": "coaps://localhost:5684/count"
+        }
+      ]
+    }
+  }
+}
+```
+
+### Client Example with PSK
+
+```js
+const { Servient } = require("@node-wot/core");
+const { CoapClientFactory } = require("@node-wot/binding-coap");
+
+const servient = new Servient();
+servient.addClientFactory(new CoapClientFactory());
+
+servient
+    .start()
+    .then(async (WoT) => {
+        try {
+            const td = await WoT.requestThingDescription("coaps://localhost:5684/secureThing");
+            const thing = await WoT.consume(td);
+
+            // configure PSK security
+            thing.setSecurity(
+                td.securityDefinitions,
+                {
+                    identity: "Client_identity",
+                    psk: "secretPSK"
+                }
+            );
+
+            const value = await thing.readProperty("count");
+            console.log("count value is:", await value.value());
+        } catch (err) {
+            console.error("Script error:", err);
+        }
+    })
+    .catch((err) => {
+        console.error("Start error:", err);
+    });
+```
+
+### Notes
+
+- The `identity` must match the server configuration.
+- The `psk` must match the server's configured secret.
+- Currently, only the `psk` security scheme is supported for `coaps://` in this binding.
+
 ### Server Example
 
 The server example produces a thing that allows for setting a property `count`. The thing is reachable through CoAP.