DOC: Add Welcome Bot to pymc repo

[reshamas]

Issue with current documentation:

Welcome Bot docs

• https://github.com/behaviorbot/welcome?setup_action=request
• https://github.com/behaviorbot/welcome?setup_action=request#usage

Example PR

pymc-devs/pymc-data-umbrella#192
#6602

Idea or request for content:

No response

[ricardoV94]

I don't know, I think bots like this take away from the real typed messages from other devs