Expand/clarify documentation of jinjify.py.

[kfogel]

The doc comment at the top of jinjify.py says:

This script takes two parameters.  The first is a latex file with
yaml frontmatter.  The body after the frontmatter is latex use as a
jinja2 template.  We pass the yaml frontmatter to that jinja2 template
and print the results.

I can't tell what the two parameters are, from reading that :-). It sounds like jinjify.py takes two files as arguments, but I'm not positive -- only one file is mentioned, unless "body" meant a separate file. (See also issue #6 about how we should have an example of the system in use).