I did download and unarchive WolframLanguageForJupyter-master.zip from the Github project page, but although there is a PacletInfo.m file there, still I find no actual paclet.
It works!! Not perfectly, but the fact that it works at all – indeed, pretty well at first glance – is amazing. I believe this is a major development for the Wolfram Language. Yes, the Jupyter frontend is way less sophisticated than the Mathematica front end, but the fact that millions of people are using it and developing for it is a major plus. To see a small sample of what it can do – and I literally created this in about half an hour – check out this link: HTML export of Jupyter Notebook sitting in Wolfram Cloud.