Inverse Hull Method
Inverse Hull Method / Backface Culling
The Inverse Hull Method is one of the easiest ways to get an outline around a Blender model. It is implemented by adding a "solidify" modifier and a second material with "backface culling" turned on. Under the wrench icon you can bring up the modifiers tab. Click add new modifier and add a solidify modifier.
Now add two material, one for the base color and one for the outline. You need to add a second "material slot" by clicking the plus button to the right of the material. The base shader can be whatever type you want to use, but the backface should have an emission shader because it displays as a solid color and is unaffected by lights and shadows. Make sure to check the "Backface Culling" on the backface material.
The Solidify modifier need to have "flip" checked in the normals section, which reverses the normals so that the outside surface behaves as in internal surface. In other words, it is backfacing and backface culling removes it from the scene. Next set the material offset to 1. An offset of zero is the first material slot. This is useful is you decide to add more material shaders to the object because it lets you choose which material to use for the object's outline.