talk about the new exec form of RUN (Closes #3723) implemented by #3558

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #3730 from SvenDowideit/JSON-RUN-format-docs

talk about the new exec form of RUN (Closes #3723) implemented by #3558