Add changelog

Per conversation in #60 used https://github.com/lalitkapoor/github-changes to generate CHANGELOG.md. This tool looks at tags, commits and pull requests to generate changelog. So I went through history of commits and tagged releases where version bump happened.

Take a look at tagged versions on my fork of the repo https://github.com/kate2753/hopscotch/releases. If they look good I will push those tags upstream as well.