Document how to call template_include_tag

When calling `template_include_path` inside an Erb template I failed to
realise that Rails would be unable to find my Mustache templates that I
was including with `template_include_path`, without prefixing the
partial's name with the relevant folder within `app/templates`.

I spent a good while stepping through Rails code in the debugger before
realising my mistake, so I thought being explicit about it might be
useful to others.
Merge pull request #46 from gma/helper-method-docs

Document how to call template_include_tag