Provide command to download binary releases

[Sander Maijers]

The command in README.md:

curl -s https://api.github.com/repos/Michael-F-Bryan/mdbook-linkcheck/releases/latest \
    | grep browser_download_url \
    | grep $(rustc -Vv | grep host | cut -d' ' -f2) \
    | cut -d : -f 2,3 \
    | tr -d \" \
    | wget -qi -

• Is Linux-only.
• Most importantly, requires rustc, which defeats the purpose of using a precompiled binary.

Please replace it with a simpler alternative to download just binaries. Maybe we can use uname and case switching to determine target triples, if matching host with target really must be automatic.

Update: I found this doesn't download a binary but the source code. I don't understand why you'd want that when you can git clone with fewer/smaller tools.

[Sander Maijers]

See also #61.