Add images to readme about ditherpunk.jl

[johnnychen94]

Adding an asset folder to docs/ definitely works, there is just one small thing: Julia Pkg Server will achieve everything in the master branch and ship them to the users. So if you're planning to add like 10M files, that would be a large burden. An extreme case like this is tlnagy/OMETIFF.jl#87

Upload images as comments here is a trick to work around this limitation. So I think it's probably a better option. But of course, need some more manual work if you plan to upload tens of images.

Another option is to have them in gh-pages branch and then use raw link here, e.g.,

the rawlink to https://github.com/JuliaImages/juliaimages.github.io/blob/master/dev/examples/color_channels/assets/indexed_image.png
![](https://raw.githubusercontent.com/JuliaImages/juliaimages.github.io/master/dev/examples/color_channels/assets/indexed_image.png)
<img src="https://raw.githubusercontent.com/JuliaImages/juliaimages.github.io/master/dev/examples/color_channels/assets/indexed_image.png" alt="bad_result.png" width="100">