Limit go routines

[mguentner]

https://github.com/NixIPFS/create-nixipfs-go/blob/master/create_nixipfs.go#L75

Opens too many files at once. Limit this by using a buffered chan, like so:

        maxAddRoutines := 10
        if err != nil {
                log.Fatal(err)
        }
        c0 := make(chan struct{}, maxAddRoutines)
        for _, f := range files {
                c0 <- struct{}{}
                go func(f string) {
                        AddToIPFS(sh, f)
                        <-c0
                }(f)
        }