installation instructions for github binaries

Someone brought to my attention that there aren't installation
instructions for the CI-generated binaries.  I have only looked at the
Linux one to try to run it on NetBSD (which can in general run Linxu
binaries), and it seemed straightforward.   But macOS and Windows are
probably harder.

If anyone would like to add instructions, please edit the wiki:

  https://github.com/bcpierce00/unison/wiki/CI-Binary-instructions

I put this on a separate page because it's likely going to end up too
long to avoid interfering with the flow of the high-level page.