Make call-to-action more explicit in community section

[patcon]

Re-ticketed from https://github.com/chaijs/chai/issues/1038#issuecomment-379708293

Language is important. "Involved Community" is quite passive and perhaps not as inspiring of action as it could be :)

I hope that this is a small improvement, but am totally open to other ideas! (I do feel that it's not the best as-is, and am in favour of some change.)

Thanks for your attention!

cc: @shellscape @keithamus