Add bots for github

[shraddhavp]

1. Add stale bot
2. Add issue labeller bot
3. And great bot for first time users

These bots might help the review of PRs

[shraddhavp]

@huan what are your thoughts about it ?

[huan]

@shraddhavp Thank you for the suggestion!

Could you please provide a detailed plan for how to use those bots, for example, how can we design the settings?

I'd like to learn more from the plan before we go ahead because those bots are always a powerful but a double-edged sword.