Added installation method for yarn

README.md

@@ -53,7 +53,11 @@ If you have your own server, you can attach PeerServer.
 1. Install the package:
     ```sh
     #$ cd your-project-path
+    # with npm
     $ npm install peer
+    
+    # with yarn
+    $ yarn add peer
     ```
 2. Use PeerServer object to create a new server:
     ```javascript