Consider adding paragraph in Preface outlining purpose of guide

[angela-li]

It would be helpful to include a short paragraph or bulleted list in the Preface indicating the purpose of the Curriculum Development Handbook.

I had originally thought this was a more general-purpose guide for all types of curriculum development, but it appears to be more tailored toward producing Carpentries-styles lessons. Perhaps something could be included answering the following questions:

• Who will find this guide useful?
• What is the intended use case of this guide?
• Which sections of the guide are general, and which are specific to the Carpentries?
• What does this guide not include? (and what are other resources that address these?)

[ErinBecker]

Thanks @angela-li! This are great questions. I'll take a stab at this in January.