Update CHANGELOG and README for 0.4.0 release (#130)

Release toolexec mode for packages that import errtrace.

Release v0.3.0 (#80)

Release v0.2.0 (#65)

Release v0.1.1 (#54)

Publish a patch release to address two minor issues:

- The `go 1.21.4` directive forces projects to upgrade
  so lower that.
- Use Markdown for the README so it renders nicely on pkg.go.dev.

Release v0.1.0 (#50)