docs: using npm instead of yarn

[thanhpd]

I've noticed that there are some commits mentioned migrating to npm in place of yarn. Also, yarn.lock file is currently behind package-lock.json.
My suggestions:

• Update documentation to use npm syntax for installing packages instead of yarn. Running script command from yarn is acceptable and take fewer keystrokes, so please share your thought about this.
• Delete yarn.lock file to avoid confusion.

[manekinekko]

Sounds good to me.