Get a basic README in place

[brandur]

Alright, so I don't love this, but here, aim to get a basic River README
in place for GitHub friendliness, largely via duplication.

Godoc is odd to say the least. These days it does dump a project README
automatically into godoc if one was present, but it's always a second
class citizen: it's collapsed when the user first visits the doc page,
and doesn't offer some niceties like the TOC that's generated for
doc.go. I searched around, and there's no easy compromise that gets
the best of both worlds.

So here:

• Move old README to docs/development.md.
• Make edits to doc.go so that it's more agnostic as to whether it's
  in godoc or rendered Markdown.
• Streamline the old doc.go somewhat for length. Add list of features
  that link out to our docs.
• Copy doc.go largely unedited to docs/README.md.

The trick with docs/README.md is that GitHub will still find a README
there and render it as the top level of a project, but godoc won't
find it and dump it into godoc (so we don't accidentally duplicate the
content twice in godoc).

On the way from doc.go to docs/README.md I made a couple small
tweaks for GitHub friendliness:

• Gave it an h1 of "River".
• Added badges.
• Starts with "River is a" instead of "Package River is a".
• Dedented all the headings. One thing that sucks about godoc is it
  won't render anything except an h1-level heading, but these look way
  too screamy in most Markdown renderings.
• Made all the code blocks Go-formatted. Godoc won't respect the three
  backtick syntax for codeblocks, so we can't use that there, but we
  also can't specify a language without using them for GitHub.

Obviously, this isn't a great situation. I tried the use of the goreadme
tool in #19, but that comes with significant problems that aren't
fixable without issuing them pull requests.

For now, I propose that we duplicate changes manually between the two
documents while making them, and eventually write our own tool to do the
duplication. This is easier than it sounds because we can leverage Go's
internal tool for parsing godoc.

[brandur]

Rendered:
https://github.com/riverqueue/river/blob/brandur-readme-2/docs/README.md

[brandur]

@bgentry Not calling this perfect, but wanted to lock something into place that we could start sending around to people. Mind taking a looking?

[bgentry]

Looks great to me!

[brandur]

Awesome! TY.