Add testing docs for v0.18.
Add testing docs for v0.18.

ChristopherDavenport/http4s,ChristopherDavenport/http4s,http4s/http4s,rossabaker/http4s,aeons/http4s,ChristopherDavenport/http4s,aeons/http4s,rossabaker/http4s,aeons/http4s