fix: make service worker/pwa opt-in and comment gatsby-plugin-offline (gatsbyjs#113)

* chore: comment out gatsby-plugin-offline; add message

* chore: add short link

Author: DSchau

gatsby-config.js

@@ -25,6 +25,8 @@ module.exports = {
         icon: 'src/images/gatsby-icon.png', // This path is relative to the root of the site.
       },
     },
-    'gatsby-plugin-offline',
+    // this (optional) plugin enables Progressive Web App + Offline functionality
+    // To learn more, visit: https://gatsby.app/offline
+    // 'gatsby-plugin-offline',
   ],
 }

package-lock.json

@@ -7,7 +7,7 @@
     "gatsby": "^2.0.19",
     "gatsby-image": "^2.0.15",
     "gatsby-plugin-manifest": "^2.0.5",
-    "gatsby-plugin-offline": "^2.0.5",
+    "gatsby-plugin-offline": "^2.0.11",
     "gatsby-plugin-react-helmet": "^3.0.0",
     "gatsby-plugin-sharp": "^2.0.7",
     "gatsby-source-filesystem": "^2.0.4",