Add README to build.

[timcdlucas]

Would be nice to have the README go on the CRAN splash page ala (here)[https://cran.r-project.org/web/packages/versions/index.html].

We would need to move the images into man/figures and remove README.md from .Rbuildignore and that's about it. I guess we could think about whether we want a slightly broader readme that really shows off some of zoons capabilities. It starts to overlap a lot with a vignette.

Unfortunately I can't see "anyone but me" in the assignees list...