move/duplicate the npm install section of "deploying your app" to user-guide

[kellyselden]

I was following the user-guide converting my app to fastboot. I got trolled hard by the "you have to run npm install in the dist folder" so your express server can run the modules. Probably because that section was in the "deploying your app" section, and I was still trying to get it running locally, so I hadn't read that far ahead yet.

I think this nugget should be earlier in the docs.

[bigredwill]

I believe this method will soon be deprecated. I ran into the same issue you originally opened tomdale/ember-network#10.

Check out this thread ember-fastboot/ember-cli-fastboot#246 (comment).