Add link to changelog in README

For developers not familiar with the convention of using `CHANGELOG` as
the filename for the changelog, add a direct link to the changelog in
the README.

Also updates the README to clarify which Sass syntax the tool supports
and adds better syntax highlighting to the installation instructions.

Change-Id: I1cb2b0bf9dc1fd225598b5e116ced5abe501bfa5
Reviewed-on: https://gerrit.causes.com/30693
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>