Documentation: include command for generating a pdf

[fredcallaway]

I struggled for a long time trying to generate a pdf with panzer foo.md -o foo.pdf before realizing that the command should be panzer foo.md -o foo.tex. It might be worth including that information somewhere for other wandering souls :)

[msprev]

Thanks -- I'll add a note about this. It's a side effect of 'pdf' being a sort of quasi-writer in pandoc, but not a fully-fledged writer. This is quite confusing as most new folks don't realise there are 2 ways to produce pdfs in pandoc/panzer: i) quick-and-dirty by passing .pdf as the output extension to pandoc/panzer; ii) writing .tex and using some sort of postflight script to compile to pdf.