Show an example of calling back into platform specific code

[williewillus]

In Patchouli and Botania, we use a serviceloader interface called IXplatAbstractions, implemented once by each loader, to allow down calls from Common code back into loader-specific code. This is probably something that is needed often enough that this template should probably have an example of it.

[Darkhax]

Yeah, this is something I use a lot in Bookshelf and a few other projects so there is definitely some value in documenting this. I will talk to Jared later today about building up the wiki.