Hello, There is no documentation page for Mesh Decals ?
How to use this feature?
https://docs.unrealengine.com/latest/images/Support/Builds/ReleaseNotes/2016/4_13/image_14.jpg
Hi Loux,
Currently there isn’t any documentation on this but it’s something we want to have in the future. To give you a head start here are some of the basics with it.
To start off with here are some screenshots and then I’ll go into the setup of it all.
Left is Traditional Method of modeling in the faces and the Right is the Mesh decals.
This first image uses a basic material to show the difference between shading for these.
Next, here is a close up from the front with a rocky surface.
Next, this is the same just from a more angled view of the corner to show the difference here with shading.
So after all this, how do you use it?
When you create your geometry you’ll want to take the base mesh and cut/copy out the part that needs to be where the decal mesh will be. This will be separate geometry that closely hugs the base mesh as part of it.
I created this in 3Ds Max using one of the column meshes from Sun Temple. The material is a multi-material so that the mesh decal is a different material element than the rest of the mesh. This is important due to how the material needs to be setup.
The floating geometry also has a buffer area pulled out from its sides so that it can feather back over part of the other geometry. This can help hide more hard edges and can be extended further than my simple example here.
For the material setup, you’ll want to make sure that the material is set to be a Deferred Decal material and then using one of the Dbuffer Decal solutions is really good for getting the color, roughness, and/or normal depending on the option you enable. (You’ll also need to enable debuffer decals in the Project Settings > Rendering).
Keep in mind that this is the initial implementation and there are some caveats and limitations of the system at the moment.
I hope this helps you get started. 
Tim