Add 0.5.0 release to readme and do some cleanup.

[non]

I just pushed 0.5.0 to Sonatype, so this PR will update the version in our README.

I also linkified the contributor names, rephrased some paragraphs, and added some code blocks to other parts of the README.

I think 0.5.0 should be a candidate for where we start our MiMA compatibility requirements. Here's my proposal for what happens next:

1. @non, @tixxit, @rklaehn, and others will get a Spire branch working that uses algebra-0.5.0.
2. We test that Spire branch to be sure that we don't see major problems with our benchmarks.
3. @johnynek and others optionally do the same thing for Algebird.
4. If needed, we make changes to cats-kernel and algebra, re-release, goto (1).
5. We all agree that we are happy with cats-kernel and algebra in their current form.
6. We freeze cats-kernel in MiMA compatibility mode.
7. We freeze algebra in MiMA compatibility mode (and call it 1.0.0).
8. Profit!

(In the meantime we can merge PRs but should aim to keep MiMA compatibility with 0.5.0 unless we notice a serious design flaw that we have to fix.)

What do you all think?

[tixxit]

This all sounds great to me. And the PR looks good too 👍

[non]

One other possible change we might want to do pre-freeze is to make our law definitions consistent with Cats'. I think that style is a bit easier to read/understand (since it came later). That idea is tracked at typelevel/cats#1272.

I don't think it's mandatory that cats-kernel and cats have exactly the same type of law definitions, but I don't see a good reason to prefer our current scheme.

[TomasMikula]

This now needs to be org.typelevel.

[non]

Good catch! I'll put this in a PR asap.