Document ggschema

[ijlyttle]

I think that ggschema represents the focus of this project, where we all need to share the same (or as close to same as possible) understanding of this intermediate form.

Two things we want to do in the short-term:

• potentially reorganize some of the ggschema code to make it easier to extend
• create a Markdown file that discusses the philosophy of ggschema, and describes how to extend

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Old thinking:

I propose to write a series of articles for the pkgdown website that I hope can encapsulate all of our understanding of what we are doing and why we are doing it.

Perhaps this issue can be ongoing, with a "perpetually-open" feature branch that could get merged into master periodically.

[ijlyttle]

Newer thinking: we will use this issue for documentation on how to extend ggschema, and how things might be organized within ggschema.