diff --git a/BUILD.md b/BUILD.md
new file mode 100644
index 0000000..1f81a50
--- /dev/null
+++ b/BUILD.md
@@ -0,0 +1,6 @@
+This book is generated using [gitbook](https://github.com/GitbookIO/gitbook).
+
+== How to build the PDF?
+
+When gitbook is installed, `gitbook pdf` will generate the PDF of the book.
+