Place throttle limits in response header

[retrohacker]

Migrated from restify/plugins#80

[eps1lon]

I did some work implementing this (which was pretty straight forward).

On which branch do you accept pull requests?

[retrohacker]

Hey @eps1lon,

Thanks for doing this ❤️! PRs should be opened against the latest major release, and backported if necessary. Currently, that is 5.x 😄

[retrohacker]

Merged in #1453