Big docs & readme update

[barucden]

Okay, this one's a little bigger.

The goal is to simplify the docs: to contain only what's this package about so we can keep it up-to-date, no duplicates (rather use cross-references), simpler structure, less talking.

The biggest change in README is the addition of contributing section.

Main changes in docs:

• Menu structure is simpler
• New example for integration with Flux.jl
• Added docs for MapFun and AggregateThenMapFun
• Removed the "Working with images in Julia" page (instead, it is only a tiny section in introduction that refers to JuliaImages docs)
• Removed indices.md

I plan to do a few more smaller changes in the future (like adding more examples, simplify further, etc.)

[johnnychen94]

I didn't check every single line but it looks good to me, I also agree some of the contents here are redundant. I kept some of the old contents during the transition because I didn't think of a good replacement for them. As a non-native English speaker, I enjoy @Evizero's words a lot.

[barucden]

Yeah, the sentences read nicely. For example, the part about images in Julia was quite educational. I almost felt pity removing it. On the other hand, maintaining the documentation is perhaps harder than maintaining the code, so I aimed to cut down anything that is not necessary (in this specific case, the documentation is available elsewhere -- JuliaImages docs). Also, I think having less text is better for the users as it's easier to skim through.

Edit: Let's keep this PR open for a while so @Evizero has a chance to express his opinion.

[Evizero]

Thanks for the kind words. While it was fun writing the images tutorial page, I do think it became outdated quite a while ago. I defer to your combined judgement which has been excellent so far. Thank you for your hard work

[johnnychen94]

Very well written! The "Label-preserving Transformations" part is very educational; I believe most people are just ignorant about this.

I reckon that we should emphasize the importance of Colorant array. Many issues in using this package are due to the old habit of the plain Array{Float64, 3} data. I didn't comment on the "Complete example" section so you might need to update it as well.

[johnnychen94]

@barucden Unless requested, I don't have more comments in. With some final word tweaks, I think we're good to go. It's up to you.

[barucden]

Thanks for the feedback! I am merging it in now. We can continue to improve the documentation iteratively.

[johnnychen94]

Looks like we need a cover image here.

[barucden]

Is it required? I could not think of any since the operations are really general.

[johnnychen94]

No, but I guess it somehow doesn't look very nice if using the fallback cover... I really have a bad taste in designing the icon/cover. :(

[barucden]

I will try to add a better cover picture in near future