Update _references.md

Created references for Design Center to distinguish between learning how to use the DC and "DC References." Also, the title of the Design Center (manuals-design-center.html) is changing so this is a good time to create a reference name for it.
Merge pull request #33 from deb-cfengine/patch-1

Update _references.md
Autocommit: syntax map changed 2014-01-06-15:51:50
Autocommit: libraries changed 2014-01-06-15:51:50