Document bedrock default sass symlinking

[mpgeek]

In https://github.com/opensourcery/vista-campus/commit/641a035, we are adding symlinks to the layouts directory to support default styles for the bedrock layouts. In the context of turnip, we will be generating those symlinks when the theme is spun up in make-install-profile. We probably need documentation of the what/how/why if OS-ACE is used outside of Turnip.