Describe nuances of coercion in docs

[bajtos]

Based on #304 (comment):

The documentation in these tests would also be helpful to everyone. I think we should create a section about coercion in the readme that lists out the nuances of coercion.

[bajtos]

I don't think this is going to happen, considering that LoopBack 4.0 GA has been released and the REST layer was rewritten completely.