Improve documentation for including XML files within each other

[gonuke]

It is possible to include information from one XML file in another XML file to build a complete input file for Cyclus. This capability has been demonstrated by a few users (e.g. https://github.com/arfc/predicting-the-past/tree/master/cyclus_input) but is not intuitive and should be better documented.

[FlanFlanagan]

I think this exists in an earlier issue. Issue #182. I've already pinged @bam241 about it, but hopefully will be taking care of this shortly if he cannot.

[bam241]

yes I agree this is a duplicate of #182

I pinged @katyhuff and @jbae11 on the other one to figure out what is their solution and which one is the most elegant one...
when this is figured, I'll be happy to update the documentation....