To use: put the WorldObject prefab under your avatar. It should be next to Armature and Body. Replace the cube with your desired object.
Use the sample "ActivateWorldObject" animation to activate the world object.

A sample override controller called "SampleWorldObjectOverrides" is included. You can use this if you want a working example first. This controller might be broken
if you do not have the VRChat SDK imported.

You can put the world object prefab somewhere else in your avatar if you like, but you will have to edit the animation paths to suit your needs.

UPDATE v2: I switched to using a humanoid model as joint object instead. For some reason this makes it work in Unity 2017 again. You might get less jittering if you
put your object under the Hips of the prefab's armature, rather than directly under the joint object.