Export a custom rig with animation from Maya to Unity

From Ingo Clemens: This tutorial is about how to transfer a custom rig including animation from Autodesk Maya to a game engine such as unity. The main problem for this process is that most of what makes up a character rig, such as controls, constraints, connections, expressions will not always get properly imported into the game engine. One way to get around this is to build a rig that includes only the features that can be read by the game engine, which are usually joints and maybe ik controllers. The downside of using such a limited rig for animation is that the animation process itself becomes a real pain because all the nice controls that can make animating a breeze are missing.