Update install instructions in README.md

[weiglszonja]

          Good question; the biggest concern would be `ndx-holographic-stimulation`, which I don't think has been officially released yet in any capacity

Technically an official release of this ought only to rely on other pip packages instead of relying on a git install to a specific commit there, but I also don't know how much longer that is going to be

Originally posted by @CodyCBakerPhD in #5 (comment)