Add a changelog

[polarstoat]

Please could you implement a changelog to keep track of changes made between versions.

Over the last month there have been 17 versions of js-base64 published, and scrolling through the commit history is a rather slow and tedious way to identify changes.

More information and suggested formatting of changelogs can be found at Keep a Changelog.

[tomchentw]

For anyone interested in the change from v2 to v3, here's the link of the GitHub compare for 2.6.3...3.3.2.