README has stale versioned links

[quasicomputational]

Not submitting a PR (yet) because it's not quite obvious what the Right Thing here is.

If the links were made relative (and the examples were put into extra-doc-files), we would be doing the right thing: sdists, installed versions, browsing the repository on GitHub, etc would all be linking to appropriate versions. But relative links will break in Hackage's README display (I think? I haven't actually checked).

If we go with explicitly versioned links (i.e., just bump the version), then we've got to remember to bump all of them with each release, and HEAD will be pointing to the last released version, which might not be current.

If we go with unversioned links, then sdists, installed versions and Hackage will have links pointing to potentially bleeding-edge examples that won't work with the released versions.