Documention on template function library

With the majority of template library functions moving into hashicat, it would be good if the downstream applications had decent reference documentation to refer to. Currently the template functions have basic docs, but nothing compared to the more extensive docs as they current exist in consul-template.

I'd like to hit somewhere between the verbosity of the current consul-template docs and the brief hashicat docs. I thinking the standard gdoc format with examples should work as a good starting point.