Use GitHub Releases for showing changes

When clicking through to the script package from Atom you are first taken [here](https://atom.io/packages/script). It would then be quite useful that when one clicks on the "Versions" link on that page, that the changes for each release are shown under your [GitHub "Releases" page](https://github.com/rgbkrk/atom-script/releases). See for example the [file-icons package](https://github.com/file-icons/atom/releases).

(As you currently only have an out of date `CHANGELOG.md`, you have to set up a [compare](https://github.com/rgbkrk/atom-script/compare/v3.17.3...v3.18.1) to see what the changes actually are.)