📝 add some descriptions in README.md

- how to install
- how to acquire keys to use exchange API

Author: wonism

README.md

@@ -1,6 +1,11 @@
 # Upbit API Node
 > Api wrapper for Upbit
 
+## Install
+```
+$ npm i -S upbit-api-node
+```
+
 ## Quotation APIs
 ### subscribe(options) => ws
 Get websocket instance to subscribe websocket protocol
@@ -80,6 +85,8 @@ const upbitExchange = new Exchange(ACCESS_KEY, SECRET_KEY);
 | accessKey                | String   | n\a                                                          |
 | secretKey                | String   | n\a                                                          |
 
+- You should go to [Open API Management page](https://upbit.com/mypage/open_api_management) to acquire these keys.
+
 ### upbitExchange.getMyAssets() => Promise
 Get user's asset information