[ci skip] Updated README / docs for release 0.7.0.

Also fixed a typo in `docs/releases/latest.rst` (`CANDIDATES`
was spelled `CANDIDES`).

[ci skip] Updated README / docs for release 0.6.4.

[ci skip] Updated README / docs for release 0.6.3.

[ci skip] Updated README / docs for release 0.6.2.

[ci skip] Updated README / docs for release 0.6.1.

[ci skip] Updated README / docs for release 0.6.0.

[ci skip] Updated README / docs for release 0.5.0.

Cutting release 0.4.0.

In the process, also removing badges from RTD docs (since they
are only meant for the "latest" build).

Cutting version 0.3.0.

Cutting release 0.2.1.