Add doc: to opam file

Author: Jeremie Dimino

spawn.opam

@@ -4,6 +4,7 @@ authors: ["Jane Street Group, LLC <opensource@janestreet.com>"]
 homepage: "https://github.com/janestreet/spawn"
 bug-reports: "https://github.com/janestreet/spawn/issues"
 dev-repo: "git+https://github.com/janestreet/spawn.git"
+doc: "https://janestreet.github.io/spawn/"
 license: "Apache-2.0"
 build: [
   ["jbuilder" "build" "-p" name "-j" jobs]