Misc. README improvements, including installation and usage info

[daveyarwood]

It took me a while to figure out how to install and use sxhkd. After skimming through the issues and finding #24, I saw comments from @baskerville explaining what dependencies are needed. I think it would be super helpful if this information were available in the README, so here's a PR!

I added various formatting improvements and usage info while I was at it. Feedback welcome.

[dogweather]

Thanks - great idea!

[daveyarwood]

I've merged in the latest master and resolved conflicts.

@baskerville I still think it would be helpful to add this information to the README. Any objection to merging my PR?

[pataquets]

@baskerville: Friendly ping. I'm sure many newcomers had stumbled on this.

[daveyarwood]

@Ascenio You approved this PR - does that mean you have push access to the repo? Maybe you could merge this PR?

[daveyarwood]

I merged master in again and resolved conflicts. 4 years later, I still believe the README would benefit from my improvements, most importantly, the setup instructions.

@baskerville If you have a couple minutes, could you please review and merge my PR? Thanks!

[Ascenio]

@Ascenio You approved this PR - does that mean you have push access to the repo? Maybe you could merge this PR?

No, I don't have any. I just took a look and thought it was ok xD