Document the manual installation of Linux binaries

[artob]

Use case

As a Linux user, until binary packages (asimov-packaging#10) are available for my distribution, I would most likely prefer to install from pre-built release binaries so long as the process isn't too long or complicated.

Proposal

Document in a README section the manual downloading and installing of relevant CLI binaries (asimov, asimov-module, asimov-snapshot).