Feature: Include Mention of Tera for Templating in Documentation

[daxpryce]

Description
It took me longer than I'd have liked to figure out that the templating in our comtrya manifests was based on tera. In my defense, I didn't already know it was a jinja-esque templating engine!

It took me a long while to figure out how to wrap up a desired comtrya manifest using the information in os.version. Long story short, installing rpmfusion free and nonfree repos can be done in a single action, but I didn't want to have to give a hoot when 44 or 45 come out and my manifests were written during 43. Anyway, I accomplished my goal - but a link to Tera's documentation and builtins would be great to have!

Manifest Example

# os.version returns something like 43.0.0 but rpmfusion's path needs `43` or `42` or , presumably one day, `44`
actions:
  - action: package.install
    where: 'os.distribution == "Fedora"'
    list: 
    - 'https://mirrors.rpmfusion.org/free/fedora/rpmfusion-free-release-{{ os.version | split(pat=".") | first }}.noarch.rpm'
    - 'https://mirrors.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-{{ os.version | split(pat=".") | first }}.noarch.rpm'

I'd add it myself and open a PR but I don't know enough to say what kinds of builtins are available vs. not available or whatever. At this point all I know is split and first seemed to work :)