adding R README file
ADD
tests/r/simple/README.rst
+R environment - install.R
+-------------------------
+
+You can install an R environment with the following two files:
+
+* ``install.R``: a script that will be run from an R installation. This is
+ generally used to install and set up packages.
+* ``runtime.txt``: include a line that specifies a date for the appropriate
+ MRAN repository version for packages. It should have the structure
+ ``r-YYYY-MM-DD``.

adding R README file