Update desktop build instructions fixes #2792

This is based on what I had to do to get the desktop app running, as per the advice in #2792. I am not familiar with `npm`, so please change this pull request if there is a more appropriate way of achieving the same result.

README.md

@@ -104,6 +104,7 @@ To run as a desktop app:
 ```
 npm install
 npm install electron
+npm run build
 node_modules/.bin/electron .
 ```