Changelog formatting tweaks

[kaelig]

Thanks for including release notes and tagging releases in GitHub!

This PR introduces these changes to the changelog:

• reorders versions to show the latest at the top
• Adds links to unreleased changes and commit log between 0.4.0 and 0.5.0
• Adds a link to the keepchangelog.com guidelines in comments
• reformat markdown to follow above guidelines

If you have questions about the [x.x.x] link notation, let me know!

[Unreleased]

This link (commented for now) allows you to add pending changes that aren't released yet.

e.g.

## [Unreleased]
### Changed
- Something changed

...
...
[Unreleased]: https://github.com/dequelabs/axe-webdriverjs/compare/v0.5.0...master

Then, when you're ready to release the next version (e.g. v0.6.0), you replace Unreleased with the version name (and update the link at the bottom of the file). This is very handy to track changes between versions.

Again, if anything is unclear, let me know.

[CLAassistant]

All committers have signed the CLA.

[marcysutton]

Thanks for your contribution!