Add a noscript tag for better DX in develop mode with JS disabled (#566)

Fixes #374

lib/utils/develop.js

@@ -25,6 +25,14 @@ import globPages from './glob-pages'
 import webpackConfig from './webpack.config'
 const debug = require('debug')('gatsby:application')
 
+// Display a nice noscript message in dev mode explaining that server-side rendering
+// is not enabled in develop mode
+const devNoScript = `<noscript>
+  The Gatsby development server does not work without JavaScript enabled.
+  If you'd like to test how your site works without JavaScript, first build the site
+  'gatsby build' and then serve the built site 'gatsby serve-build'
+</noscript>`
+
 function startServer (program) {
   const directory = program.directory
 
@@ -71,7 +79,7 @@ function startServer (program) {
           try {
             const htmlElement = React.createElement(
               HTML, {
-                body: '',
+                body: devNoScript,
               }
             )
             let html = ReactDOMServer.renderToStaticMarkup(htmlElement)