Add behaviorbot to repo to add welcome messages

[matthewfeickert]

https://github.com/behaviorbot allows for automatically responding to new users and can give some helpful advice if people don't follow templates.

It would be good to enable behaviorbot on pyhf and to add a .github/config.yml to control it.

For examples of how this is done (at the org level) c.f. jupyterhub's at https://github.com/jupyterhub/.github/blob/main/.github/config.yml

[matthewfeickert]

Example of a first PR interaction: jupyterhub/binderhub#1659

[matthewfeickert]

c.f. this (similar) example from Gammapy: https://github.com/gammapy/gammapy/blob/d44a1c240dfab9bf9a44e7ce066cc534f3ea6365/.github/workflows/greetings.yml