readme: add link to huburis

There was a link to Humility in a blog post, and since it wasn't the point of the blog post that post did not explain what Humility was. So I clicked that link and came to this repo. This repo's readme says that it's a debugger for Hubris and assumes you know what Hubris is, which I would argue is hubris in and of itself. I had no idea what it was. I took a lucky guess that it would be another project on the same org and was correct. I think that hyperlink to that other project would be both an easy and natural way for others to not have to make that same guess.

Author: will