Add icons to gatsby-plugin-manifest example options (#1140)

* Add icons to example options, mention the "static" directory

* Fix path to 'static' folder

packages/gatsby-plugin-manifest/README.md

@@ -26,14 +26,29 @@ plugins: [
   {
     resolve: `gatsby-plugin-manifest`,
     options: {
-      "name": "GatsbyJS",
-      "short_name": "GatsbyJS",
-      "start_url": "/",
-      "background_color": "#f7f0eb",
-      "theme_color": "#a2466c",
-      "display": "minimal-ui",
+      name: "GatsbyJS",
+      short_name: "GatsbyJS",
+      start_url: "/",
+      background_color: "#f7f0eb",
+      theme_color: "#a2466c",
+      display: "minimal-ui",
+      icons: [
+        {
+          // Everything in /static will be copied to an equivalent
+          // directory in /public during development and build, so
+          // assuming your favicons are in /static/favicons,
+          // you can reference them here
+          src: `/favicons/android-chrome-192x192.png`,
+          sizes: `192x192`,
+          type: `image/png`,
+        },
+        {
+          src: `/favicons/android-chrome-512x512.png`,
+          sizes: `512x512`,
+          type: `image/png`,
+        },
+      ],
     },
   },
 ]
 ```
-