Update README.md

@@ -57,7 +57,16 @@ git clone http://khuhub.khu.ac.kr/2019102240/Straight-Up.git
     yarn
 ```
 
-4. Start the Program   
+4. Get API key
+    > KaKao Pose Api key를 획득해 API_KEY에 입력합니다.
+[Pose API][poseapilink]
+[poseapilink]: https://developers.kakao.com/product/pose
+```
+    // Main.js
+    const API_KEY = /* your API key */
+```
+
+5. Start the Program   
     > **`npm start`** 입력 후 **localhost:3000**으로 접속합니다.
 ```
     npm start