Update README with secret field documentation

- Document secret parameter in offer creation examples
- Add Protected Offers section with detailed usage
- Update API reference for create() and answer() methods
- Show hasSecret flag in discovery responses

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "@xtr-dev/rondevu-client",
-  "version": "0.7.3",
+  "version": "0.7.6",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@xtr-dev/rondevu-client",
-      "version": "0.7.3",
+      "version": "0.7.6",
       "license": "MIT",
       "dependencies": {
         "@xtr-dev/rondevu-client": "^0.5.1"