Copy-paste commands to easily download and install binaries

[AustinWise]

Some ideas are in the CLI book: https://rust-cli.github.io/book/tutorial/packaging.html

trust gets most of the way there. There are some tweaks I would like to make:

• Instead of using rustc to detect the target triple, steal the target detection code from the rustup-init.sh script
• Instead of parsing the message GitHub gives when doing a 302 redirect to find the the latest version, do something else. Either a lot of work to discover the latest version at download time, or include the version number in the install instructions.
• Add a PowerShell script for Windows users

Also, a cargo install command should be listed, so people can install on targets I have not yet create binaries for.

Some more pie-in-the-sky ideas:

• rustup already does a lot of what I want for binary distribution and installation. Perhaps a binary rust package manager could be built out of parts of it?

[AustinWise]

See also how the rustup website detects the OS, to suggest which installer to use: https://github.com/rust-lang/rustup/tree/master/www

[AustinWise]

I had an idea:

• Instead of parsing the message GitHub gives when doing a 302 redirect to find the the latest version, use include a version.txt as one of the release files. See 769fc8d

Unfortunately there does not appear to be a URL that will allow us to reach this file =( .