add hints for R testing

* default.mk: Add hints for what is required to test with ob-R enabled.

default.mk

@@ -33,12 +33,13 @@ BTEST_PRE   =
 # add options after standard load path
 BTEST_POST  =
               # -L <path-to>/ert      # needed for Emacs23, Emacs24 has ert built in
+              # -L <path-to>/ess      # needed for running R tests
               # -L <path-to>/htmlize  # need at least version 1.34 for source code formatting
 BTEST_OB_LANGUAGES = awk C fortran maxima lilypond octave python sh
-              # R is not activated by default because it requires ess to be installed and configured
+              # R                     # requires ESS to be installed and configured
 # extra packages to require for testing
 BTEST_EXTRA =
-
+              # ess-site  # load ESS for R tests
 ##->8-------------------------------------------------------------------
 ## YOU MAY NEED TO ADAPT THESE DEFINITIONS
 ##----------------------------------------------------------------------