Add documentation for Palette shared binding

[matt-land]

Add documentation for the palette subscript operator and how to use it.

[ladyada]

i think you can also set it to None if you want transparant

[ladyada]

lgtm but ill let @tannewt do another check in case i missed something :)

[tannewt]

None doesn't currently mean transparent. It's not a bad idea though.

For now I think it will throw an error so please update the docs for as it is now.

[tannewt]

Thank you!