docs: update README.md

Author: jbl428

README.md

@@ -17,7 +17,7 @@ This library is inspired by the HTTP interface in Spring 6 and provides a simila
 - Provides a concise syntax for handling query parameters, path variables, request headers, request bodies, and forms.
 - Offers integration with [class-transformer](https://github.com/typestack/class-transformer) to facilitate data
   transformation.
-- Uses promises instead of observables
+- Support both promise and observable.
 - Circuit breaker support
 
 ## Requirements
@@ -118,6 +118,9 @@ class UserService {
   options of the circuit breaker. You can use global options by setting the `circuitBreakerOption` property in the module.
   `options` is from [opossum](https://www.npmjs.com/package/opossum) library.
 
+- `@Observable()`: Marks the method as an observable method. If this decorator is not specified, the method will return
+  a promise.
+
 ## Auto generate `@ResponseBody()` from return type of exchange method
 
 Because of limitation of `reflect-metadata`, `@ResponseBody()` is required to specify the response DTO.