Despite having a big warning at the bottom of the docroot/sites/default/settings.php file telling you not to add additional settings to that file. The BLT documentation does little do say what the recommended way of conditionally including *.settings.php files depending on environment variables or some other decider. Normally, you would do it by adding code to the effect of the below to docroot/sites/default/settings.php but since there is such a direct warning not to in the BLT version, I needed to find a better way: