feat: add install instructions for external commands

Issue reported on IRC.

core/commands/external.go

@@ -11,7 +11,7 @@ import (
 	cmds "github.com/ipfs/go-ipfs-cmds"
 )
 
-func ExternalBinary() *cmds.Command {
+func ExternalBinary(instructions string) *cmds.Command {
 	return &cmds.Command{
 		Arguments: []cmds.Argument{
 			cmds.StringArg("args", false, true, "Arguments for subcommand."),
@@ -27,7 +27,7 @@ func ExternalBinary() *cmds.Command {
 						buf := new(bytes.Buffer)
 						fmt.Fprintf(buf, "%s is an 'external' command.\n", binname)
 						fmt.Fprintf(buf, "It does not currently appear to be installed.\n")
-						fmt.Fprintf(buf, "Please refer to the ipfs documentation for instructions.\n")
+						fmt.Fprintf(buf, "%s\n", instructions)
 						return res.Emit(buf)
 					}
 				}

core/commands/root.go

@@ -144,7 +144,7 @@ var rootSubcommands = map[string]*cmds.Command{
 	"swarm":     SwarmCmd,
 	"tar":       TarCmd,
 	"file":      unixfs.UnixFSCmd,
-	"update":    ExternalBinary(),
+	"update":    ExternalBinary("Please see https://git.io/fjylH for installation instructions."),
 	"urlstore":  urlStoreCmd,
 	"version":   VersionCmd,
 	"shutdown":  daemonShutdownCmd,