Create INSTALL.md for detailed install instructions

I think we should create an INSTALL.md file in the root of this repo with detailed installation instructions for each OS, including our recommended installation method, as well as alternatives.

For Mac, for example, we recommend the official installer on the website, but it's also available via a Homebrew Cask.

For each Linux variant, there will be a different command or preferred file to use.

Does someone want to take a stab at an initial version of this document? We can also link to it from the WebTorrent Desktop homepage, near all the download links.