net/http/pprof: updated documentation (run an http server)

Added instructions for starting an http server
to the godoc header for this package.  With the old
instructions, the example "go tool pprof..." commands
wouldn't work unless you happen to be running an http
server on port 6060 in your application.

R=golang-dev, minux.ma, adg, giacomo.tartari
CC=golang-dev
https://golang.org/cl/6483049

src/pkg/net/http/pprof/pprof.go

@@ -14,6 +14,14 @@
 // To use pprof, link this package into your program:
 //	import _ "net/http/pprof"
 //
+// If your application is not already running an http server, you
+// need to start one.  Add "net/http" and "log" to your imports and
+// the following code to your main function:
+//
+// 	go func() {
+// 		log.Println(http.ListenAndServe("localhost:6060", nil))
+// 	}()
+//
 // Then use the pprof tool to look at the heap profile:
 //
 //	go tool pprof http://localhost:6060/debug/pprof/heap