Adding a proposal for a recipe introduction section

[pelted]

From CONTRIBUTING.md

This PR is a proposal submission to add support for an introduction section to a recipe and to facilitate defining ingredients at the top of a .cook file.

See discussion #46

[pelted]

I was just looking at cooklang-swift and maybe it would be easier to do pre-defined ingredients with the comment syntax.

>> source: https://www.joshuaweissman.com/post/easy-authentic-thai-green-curry

== Homemade Thai Green Curry deserves respect when making it.
== Good thing it's actually very easy and just requires a little elbow grease.

-- @coriander seeds{2%tsps}
-- @cumin seeds{1%tsps}
-- @white peppercorn{1%tsps}

First step goes here