Add threading model documentation (#183)
Add threading model documentation (#183)

* Add threading model documentation

* Apply suggestions from code review

Co-authored-by: Geoff Hubbard <abef9b7bb2ce26e0dffb16b964465d3810617e62@users.noreply.github.com>

* Move documentation in README.md

Co-authored-by: Geoff Hubbard <abef9b7bb2ce26e0dffb16b964465d3810617e62@users.noreply.github.com>
DataDog/dogstatsd-ruby