Document each for extending collections.

[nathan-at-least]

Request

Document collection extension with each directive.

Next Steps

I may submit a PR with the documentation as I learn how each works. I've read #287 and then discovered the expansion definition.

Background

I just discovered derive_builder yesterday after an hour of tediously hand-writing multiple builder types. This is a very useful tool!

I converted every builder in my project to use derive_builder except for a builder that needed to extend a BTreeMap. When I came this issue tracker to see if others had asked for this support, I found #287 and realized there's already functionality for this. I went back and read over the crate docs, but couldn't find any detail on that.

[TedDriggs]

A PR with a documentation update and/or an entry in examples would be welcome.