boilerplate for r-base image documentation
Still needs a proper "how to use this image" section;
this just fills in the license, logo, wikipedia summary etc
one set of edits
add a few more realistic examples of use cases
use the new default CMD in the example use
update Dockerfile links
Add license link and correct CMD example as suggested by @tianon
Merge pull request #110 from rocker-org/master

Docs for the r-base image