fix instructions for building UI

"yarn build" doesn't actually fetch packages. When starting from a clean
client, you have to run "yarn" (with no arguments) first.

guide/install.md

@@ -84,6 +84,7 @@ For instructions, you can skip to "[Camera configuration and hard disk mounting]
 
 Once prerequisites are installed, Moonfire NVR can be built as follows:
 
+    $ yarn
     $ yarn build
     $ cargo test
     $ cargo build --release