Merge spawn.descr into spawn.opam

Signed-off-by: Jeremie Dimino <[email protected]>

spawn.opam

@@ -17,3 +17,22 @@ depends: [
   "ppx_expect" {test}
   "ocaml" {>= "4.02.3"}
 ]
+synopsis: "Spawning sub-processes"
+description: """
+Spawn is a small library exposing only one functionality: spawning sub-process.
+
+It has three main goals:
+
+1. provide missing features of Unix.create_process such as providing a
+working directory
+
+2. provide better errors when a system call fails in the
+sub-process. For instance if a command is not found, you get a proper
+[Unix.Unix_error] exception
+
+3. improve performances by using vfork when available. It is often
+claimed that nowadays fork is as fast as vfork, however in practice
+fork takes time proportional to the process memory while vfork is
+constant time. In application using a lot of memory, vfork can be
+thousands of times faster than fork.
+"""