docs: embed usage example under unreleased #25 entry

Author: ytkg

CHANGELOG.md

@@ -9,6 +9,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 - Allow `Switchbot::Client.new` to read `SWITCHBOT_API_TOKEN` and `SWITCHBOT_API_SECRET` when arguments are omitted. ([#25](https://github.com/ytkg/switchbot/pull/25))
+  Usage:
+
+  ```ruby
+  ENV['SWITCHBOT_API_TOKEN'] = 'YOUR_TOKEN'
+  ENV['SWITCHBOT_API_SECRET'] = 'YOUR_SECRET'
+  client = Switchbot::Client.new
+  ```
 
 ## [0.8.0] - 2023-03-21