Add style guide to docs/readme so new contributors can be aware of UI/design

[heartsucker]

Basically take the notes from #1536 and other recent issues and compile them somewhere central so when forgetful people like me try to make changes 6 months from now, there will be a convenient place to look.

[psivesely]

I think making it into a checklist w/ sub-issues is probably the way to go.

[ninavizz]

@heartsucker I love you for creating this issue! A UX styleguide is a project unto itself, however. Global IxD patterns and UI styles are important to have documented. Even when done #jamecono style tho, it's a proper project. I def agree tho, we gotta start somewhere—but I'd like to chat with the team about how best to get it going. @justintroutman cc'ing you here as FYI. :)

@fowlslegs Ya, my Source 0.4 issue that I posted yesterday, is kinda my initial jab at that. I've never even seen the Journo or Admin experiences tho, so between the three, I'm sure there's a lot to document, then work towards making consistent across all three. The first page from the PDF file I had in that bug, seems like a good starting point for tweaking towards a first step:

[psivesely]

Digression: we love @heartsucker at FPF. Dude does so much good work; it's awesome to have him as a contributor. Deserves some public recognition.

While I'm at it, you've also been doing awesome work @ninavizz, and we're really grateful SD is getting some long overdue attention from a skilled UI/UX designer such as yourself.

[justintroutman]

@ninavizz I'm totally on board with the importance of a style guide. Let's set aside a day this week to talk more about this, and the other UX items.

[ninavizz]

Created #1552 as a stopgap interim measure.

[eloquence]

This is still relevant, but very outdated; since this issue was filed, a lot of work has gone into documenting UI assets and styles, but there is not yet a single canonical styleguide. See https://github.com/freedomofpress/securedrop-ux/wiki for current state.

[eloquence]

Upon consideration, closing in favor of more up-to-date UX tracking issue here: freedomofpress/securedrop-ux#65