contributing: add information for caine bot

[indutny]

Caine is a nice bot that should help us tag issues.

See https://github.com/indutny/caine for details.

[indutny]

cc @mikeal please check wording.

[rvagg]

I'm not sure what the answer to this is but I wouldn't mind the focus to be lifted from the "core team" a little bit to contributors--remember that it's not only the TC now but we need to continue to encourage participation from everyone outside the TC who has commit access because of the open structure.

Maybe we could get a list of committers on the README on in CONTRIBUTING along with areas that they nominate themselves as being interested in, at least calling them out would go a long way to promoting the open structure and encouraging more people to get involved because they can see a path to sharing in the project.

[indutny]

@rvagg well, this CONTRIBUTION.md changes are exactly for this. You could put your name next to one of the subsystems, and you'll be randomly selected from the list every time the new issue appears.

[indutny]

Landed in 823cd54, thank you!