Download mdBook if it doesn't exist

build

@@ -3,6 +3,16 @@
 set -euxo pipefail
 
 rm -rf public/
+
+if [ -x $(which mdbook) ] ; then
+    echo "mdBook found, running..."
+else
+    echo "mdBook not found, downloading..."
+    wget https://github.com/rust-lang/mdBook/releases/download/v0.4.13/mdbook-v0.4.13-x86_64-unknown-linux-gnu.tar.gz -O mdbook.tgz
+    tar zxvf mdbook.tgz
+    mv mdbook /usr/local/bin/
+fi
+
 mdbook build -d public
 
 # Make checkboxes editable, just in case the user wants to keep their own checklist.