review and update.

[MichaelDimmitt]

split subfolder out into a new repository.

https://help.github.com/articles/splitting-a-subfolder-out-into-a-new-repository/

http://www.nomachetejuggling.com/2011/09/12/moving-one-git-repo-into-another-as-subdirectory/

instead, submodule

https://git-scm.com/book/en/v2/Git-Tools-Submodules