Add some code documentation

I'd appreciate if you add a comment in the file head describing the logic starting from the entry point (`DeepOverrideHost.addProperty`) and briefly explaining the algorithm.

Something like this:
1. The entry point -- `addProperty`, accepts this and that parameter
2. Then we're building the object state tree for the given path (`buildAbstractStateTree`)
3. Using this tree we're applying the object state and override necessary properties (`applyObjectState`)
4. Etc, etc

This will greatly help the reader to understand the code.