chore: update readme, contributing.md, and fix typos

[SethFalco]

• Add an EditorConfig file so it's easier code editor settings don't get in the way while contributing to the project.
• Fix typos in CONTRIBUTING.md, also the link fragment in one of the URLs was outdated. (setup vs set-up)
• Docker gives warning about apt vs apt-get, could we just make this user apt-get so we don't have to deal with them? ^-^' (Plus I guess it's better to go the stable API route anyway.)
• Very minor code change, just removes a redundant import and makes nouns like Discord/Matrix match the proper capitalization.
• While setting up the bridge, I added the 2 config properties from the README, but then the bot failed to start becasue I had 2 remove those 2 config properties. If they're outdated, I guess they shouldn't be suggested on the README either?
• Update broken link in comments.

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These are all just small things I found while working on https://github.com/matrix-org/matrix-appservice-discord/pull/854. I didn't do a CHANGELOG entry intentionally since these are just chores, let me know if you want me to add one anyway.

[Miepee]

Docker gives warning about apt vs apt-get

Why does Docker give warnings? AFAIK apt-get is deprecated, so programs should use apt whenever possible

[SethFalco]

apt-get isn't deprecated! In scripts, apt will throw a warning because it's not considered a stable interface.

So for scripts/Docker, it's recommended to use apt-get.

https://askubuntu.com/a/829869/938482