chore(docs): add keybindings in README for which-key users

[chardskarth]

I added these for better DX, because I don't think it's possible for devs to remember all these available feature. With which-key, the user should be able to see what each keybinding does.

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This plugin is awesome, and very helpful.

I sure hope a lot of people will contribute to your project @harrisoncramer !