Include info about configuring shortcut keys about rearrange-tabs

[mohnish]

@tnorthcutt That's correct. Chrome has a requirement to use one of 'em in the extension shortcuts.

[tnorthcutt]

@mohnish makes sense, thank you!

[mohnish]

Probably even change the default shortcut? Something to think about in order to avoid the confusion.

[tnorthcutt]

Is there a reason that the shortcuts must require one of control, command, or alt (on macOS)? I think I'd like to set a shortcut that doesn't necessarily use one of those, but not sure if that's a Chrome restriction rather than a UX choice you made.

[mohnish]

Addressed in #52