Make quickstart guide more friendly

[2kindsofcs]

What would you like to be added:

• a full working example of code
• maybe FAQ

Why is this needed: I believe the quickstart guide should contain a full working example. so that newcomers can easily give it a try.

the day before I tried to follow the steps described in yorkie quickstart. I created a simple file, "index.js", and wrote example code, then ran the file with "node index.js" as I used to run files with node on my local. it is no surprise that an error popped up.

ReferenceError: self is not defined
    at Object.<anonymous> (/Users/saemi.park/WebstormProjects/yorkie-test/node_modules/yorkie-js-sdk/dist/yorkie-js-sdk.js:32:4)

later the sentence You need an environment that can run JavaScript, such as a browser. rang a bell to me. if a user can just copy & paste the example code and see it working, it would be more helpful.