How to cast a shadow onto an invisible quad/plane?

I've seen this answer here : http://answers.unity3d.com/questions/253199/invisibletransparent-plane-receiving-and-rendering.html

but I'm such a n00b that I don't know what to do with that code. It looks like a shader program; but I'm not sure..

So the plane is totally transparent, I've tried loads of shaders, and they work in that I can get shadows; but the shadow is always the same transparency as the rest of the texture.

what I need is that the texture maintains the shadow at 50% alpha and the rest of the texture is 0% alpha...

Any one got any ideas? Or better still a snippet of code, or asset store link?