Add contributing doc and theme template

[hoonweiting]

Hi! I've created the first draft of the contributing doc and the theme template. I'm not super sure where the template CSS file should go, in the root directory or in some other directory.

Some notes:

• I added @RunDevelopment as co-author of the first commit because I borrowed some stuff from Added themes.json and some extras #92!
• I'm setting this PR as a draft for now. When writing is involved I usually like to sit on it for a while, and see whether it makes sense again after a while (especially the comments in the template!). But I also hope for this to be a collaborative effort, so I would love to hear everyone's feedback on how the docs can be improved!
• The README has yet to be updated.
• I'm using the One Dark colours in the template because I couldn't stand stylelint giving me all those warnings when I left them empty, haha. However, I don't know if this is the best decision, since it can seem opinionated (and One Dark has overrides that aren't just colour). Edit: Changed to unset
• Not done with the overrides for the Treeviewer plugin, haven't poked around enough there. Edit: Probably not going to bother

[RunDevelopment]

Awesome work @hoonweiting!

I have a few suggestions.

[hoonweiting]

I think I'm happy with the docs? Not super sure about the README, the contributing subsection feels a little too short and doesn't mention other ways to contribute!

Also added the selectors for overriding the Treeview plugin.

[RunDevelopment]

Sorry for the delay @hoonweiting!

Apart from my comments, I think this is pretty much ready as-is. Just a few minor nits.

Not super sure about the README, the contributing subsection feels a little too short and doesn't mention other ways to contribute!

I think it's fine. A link to the contributing docs is enough and those docs mention more than we ever had.

[hoonweiting]

Mm I'm thinking of adding lines like "You are free to add more properties that aren't stated here, such as text-shadow" and "If you wish to style the plugins, please grab their selectors from their respective .css files in the template folder", but I'm not sure if it'll be too nitty-gritty!

[RunDevelopment]

Yeah, I was thinking about adding something like this too. Maybe just a sentence or two that they are free to add more properties & rules and support for plugins.

[RunDevelopment]

LGTM; Anything else you would like to add or can I merge this?

[hoonweiting]

I'm good! I mean, I did think about maybe adding another line that tells the reader to delete the instructional comments, but I also think that would be in far-too-detailed territory. Then again, as the person who wrote it, it is probably obvious to me that those instructions can be deleted, whereas an actual user could be completely oblivious. It is hard to make a sound judgement with zero user feedback.

[RunDevelopment]

I agree. Let's get some feedback on this before we make any further additions.

Thank you for contributing @hoonweiting!