introduce `gray`, `red`, `blue`, `green` in quickstart

[johnnychen94]

Some folks come up with questions on how to extract one channel; the solution is simple: red.(img). I believe we should add these to the quickstart page.

[Jay-sanjay]

sure I will add it , please to provide the path to the code of red.(img)

[ashwani-rathee]

Here: https://github.com/JuliaGraphics/ColorTypes.jl/blob/6a10da4e309ade78dcf8e6def510ca0cb3684901/src/traits.jl#L14

[Jay-sanjay]

sir does the quickstart page refers to the get started page