Add CHANGELOG

To ensure we start capturing the history of the tool early on, introduce
a CHANGELOG to record the various amendments that will happen to
`haml-lint`.

Change-Id: I232fcd87cfd2883dc6a74e778fb02715c97c4287
Reviewed-on: https://gerrit.causes.com/28455
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>