r: add page (#1524)

pages/common/r.md

@@ -0,0 +1,15 @@
+# r
+
+> R language interpreter.
+
+- Start an R interactive shell (REPL):
+
+`R`
+
+- Check R version:
+
+`R --version`
+
+- Execute a file:
+
+`R -f {{file.R}}`