docs: Update README to improve "getting started"

[garrett]

krunvm is really cool.

However, the README seems out of date in a few ways:

• You don't have to use coprs for Fedora anymore.
• Other distros aren't mentioned.
• There's no guide on how to use krunvm.

The video demo does have some examples, but these should be in the README as well.

I was reminded of krunvm from this Mastodon post: https://mastodon.xyz/@[email protected]/110034266022052998 which mentions not just Fedora, but openSUSE, AUR for Arch, and new instructions for macOS M1/M2.