docs: Update directory descriptions in README

[matt-development-work]

I added header titles to each section, made some revisions to the description text, and hyperlinked each directory name to its respective url.

Please do make any additional edits to my changes if you want to.

[chrisdavidmills]

Thanks @matt-eric, this looks pretty nice. My one nitpick is — can you make the headings <h2>s? It seems a bit odd to have <h3>s sitting below <h1>s.

Thanks!

[matt-development-work]

Thanks @chrisdavidmills, that does seem like the appropriate choice. The headers are <h2>s now.

[chrisdavidmills]

Perfect, thanks!