Docs/cookbook: add an R + latex example.

This shows how to dynamically generate a plot in R+ggplot2, then embed
it into a latex document, and compile it to pdf, all with proper
autodependencies.

Documentation/cookbook/latex/all.do

@@ -0,0 +1,8 @@
+for d in latex dvips dvipdf Rscript; do
+    if ! type "$d" >/dev/null 2>/dev/null; then
+        echo "$0: skipping: $d not installed." >&2
+        exit 0
+    fi
+done
+
+redo-ifchange paper.pdf paper.ps