Add how-to to README for adding a chapter to the book

[wbryant]

It would be great to have a quick bit on how to add a proposed chapter to the book (i.e. where you "register" a .md file to be included).

[ChrisBeeley]

I wonder if we could make it easier to contribute by including a HackMD link, let people write in there and pull it together when they've finished.

This would also have the benefit of allowing live/ rapid collaboration

[Lextuga007]

I think the .md file requirement will be redundant if @tomjemmett's pull request #25 is accepted as that uses the .Rmd format which is a bit more intuitive. If you need a chapter you just create a new .Rmd file.