Import submodule for the book.

steveklabnik · steveklabnik · commit f17965da1e13 · 2017-03-20T10:10:15.000-04:00
It's all in the external repository now.
diff --git a/.gitmodules b/.gitmodules
@@ -27,3 +27,6 @@
 [submodule "reference"]
 	path = src/doc/reference
 	url = https://github.com/rust-lang-nursery/reference.git
+[submodule "book"]
+	path = src/doc/book
+	url = https://github.com/rust-lang/book
diff --git a/src/doc/book b/src/doc/book
@@ -0,0 +1 @@
+Subproject commit 979c56634f3fe594c9554c555eee29311d7d4a50
