add bookdown example
diff --git a/docs/bookdown_example/book.bib b/docs/bookdown_example/book.bib
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+++ b/docs/bookdown_example/book.bib
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+@Book{xie2015,
+  title = {Dynamic Documents with {R} and knitr},
+  author = {Yihui Xie},
+  publisher = {Chapman and Hall/CRC},
+  address = {Boca Raton, Florida},
+  year = {2015},
+  edition = {2nd},
+  note = {ISBN 978-1498716963},
+  url = {http://yihui.name/knitr/},
+}