Add sitemap plugin to www (#1541)

Author: Nicholas Young

www/gatsby-config.js

@@ -1,6 +1,7 @@
 module.exports = {
   siteMetadata: {
     title: `Gatsby`,
+    siteUrl: `https://www.gatsbyjs.org`,
   },
   mapping: {
     "MarkdownRemark.frontmatter.author": `AuthorYaml`,
@@ -94,5 +95,6 @@ module.exports = {
         trackingId: `UA-93349937-1`,
       },
     },
+    `gatsby-plugin-sitemap`,
   ],
 }

www/package.json

@@ -16,6 +16,7 @@
     "gatsby-plugin-offline": "^1.0.1",
     "gatsby-plugin-react-helmet": "^1.0.1",
     "gatsby-plugin-sharp": "^1.0.1",
+    "gatsby-plugin-sitemap": "^1.2.0",
     "gatsby-plugin-twitter": "^1.0.5",
     "gatsby-remark-autolink-headers": "^1.0.1",
     "gatsby-remark-copy-linked-files": "^1.0.1",