Clearer instructions for Windows

[twhb]

As somebody unfamiliar with Windows helping somebody use age on Windows, I hit a few stumbling blocks:

• The Installation section doesn’t mention where to get the Windows pre-built binary directly, only implies how to guess the URL. Linking to the Releases page, or listing all available architectures, would have saved me some time.
• I couldn’t tell if I’d have to switch to Windows-style / for options, or stick with -. Simply stating that there’s no change would have saved some back and forth.

If this is just me being a noob at Windows then fine, I just thought I’d mention it as a potential doc clarity improvement. I will make the doc changes and open a PR if desired.