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.

[hackerwins]

I'm also sympathizing with this issue. And we have several issues similar to this.

• Add Tutorials for Building Collaborative Applications using JS SDK #91
• Make JS SDK Reference Easy to Find in Documentation Page #98

So I'll close this issue for now.

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.

This bug might be fixed by yorkie-team/yorkie-js-sdk#378.