Improve README.md, add CONTRIBUTING.md, and more!

[BD103]

Closes #109.

This PR:

• Creates bevy_lint's README.md
• Modifies the main CLI's README.md
• Creates and moves some information into CONTRIBUTING.md
• Adds README.md into bevy_cli's rustdoc output.
• Sets up some custom CSS styles so Github alerts render nicely in rustdoc.

Todo

• Install bleeding edge from Git
• Install from a release
  • bevy_lint cannot be published due to Git dependency #150 needs to be resolved before I can write this section. Also see conversation on Discord for more context.
• Dealing with #![register_tool(bevy)], #[allow(bevy::lint)], and #[cfg_attr(bevy_lint, ...)].

[BD103]

Unfortunately Github alerts are currently broken, but I'll take a look in the morning.

[BD103]

So I tried using Github's alerts

Tip

like this

but they don't render correctly when wrappen in a <div>, turning into

[!CAUTION]

this :(

As such, I'm going to keep the pretty rustdoc alerts but replace Github's side

Note

With this. It's not nearly as nice, but I like it better than having no pretty alerts at all, or having rustdoc's plain block quotes.