Well it’s not working right now, that’s the problem 
I just created this post in an attempt to solve it: Get custom displacementMap (RGB encoded) to work with shadows in r132
Thing is: Basic shadowing is rather easy, it really depends on how you’re creating your terrain. If it is offsetted on the CPU, any out of the box shadowing should work (there are many (!) tutorials, examples, etc.). If it happens in the vertex shader, things start to get tricky …