improving readme

tests/r/simple/README.rst

@@ -1,10 +1,19 @@
 R environment - install.R
 -------------------------
 
-You can install an R environment with the following two files:
+You can install an R, RStudio, and IRKernel 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``.
+*  A ``runtime.txt`` file with the text::
+
+       r-YYYY-MM-DD
+
+   Where 'YYYY', 'MM' and 'DD' refer to a specific
+   date snapshot of https://mran.microsoft.com/timemachine
+   from which libraries will be installed.
+*  An optional ``install.R`` file that will be executed at build time and can
+   be used for installing packages from both MRAN and GitHub.
+
+The presence of ``runtime.txt`` is enough to set up R, RStudio, and IRKernel. It
+uses the ``r-base`` package from the Ubuntu apt repositories to install
+R itself.