add bookdown example
diff --git a/docs/bookdown_example/preamble.tex b/docs/bookdown_example/preamble.tex
new file mode 100644
index 0000000..dfd2e14
--- /dev/null
+++ b/docs/bookdown_example/preamble.tex
@@ -0,0 +1 @@
+\usepackage{booktabs}