Add docs for seeding RNG with seedRNG(int)

[mtfishman]

As described in this support answer: http://www.itensor.org/support/866/initialize-the-random-seed-of-mps-sites

[emstoudenmire]

Yes, good to add, although shoudn’t this be posted to ITensor/ITensor issues, instead of ITensor/website?

[mtfishman]

Hmm I dunno, should we add docs issues there or here? I guess it is more consolidated if it is over there.

[emstoudenmire]

Oh, I see this is about docs. I thought you meant about adding a new function to ITensor that doesn't exist, which is why I was saying that.