[Documentation] Generating hash strings

[ernsheong]

I think most bystanders would come here and desire to get a hash string like 6c1b07bc7bbc4be347939ac4a93c437a using Sum128WithSeed()

However, two things still need to be done to achieve that:

1. Use strconv.FormatUint to convert each number output (h1 and h2)
2. Concatenate the output

This should probably be in the README somewhere, to help newbies along :)