Add seo docs (#4766)

* add seo link

* Create seo.md

* Typo

* Typo

* Copy edits

Author: calcsam

docs/docs/seo.md

@@ -0,0 +1,26 @@
+# SEO with Gatsby
+
+Gatsby helps your site place better in search engines. Some advantages come out of the box and some require configuration.
+
+### Server rendering
+
+Because Gatsby pages are server-rendered, rather than client-rendered, all the page content is available to Google and other search engines or crawlers.
+
+(You can see this by viewing the source for this page).
+
+### Speed boost
+
+Gatsby's many built-in performance optimizations, such as rendering to static files, progressive image loading, and the PRPL pattern—all help your site be lightning-fast by default. 
+
+Starting in January 2018, Google [rewards faster sites with a bump in search rankings](https://searchengineland.com/google-speed-update-page-speed-will-become-ranking-factor-mobile-search-289904).
+
+### Page metadata
+
+Add metadata to pages, such as page title and description, helps search engines understand your content and when to show your pages in search results.
+
+A common way to add metadata to pages is to add [react-helmet](https://github.com/nfl/react-helmet) components (together with the [Gatsby React Helmet plugin](/packages/gatsby-plugin-react-helmet) for SSR support) to your page components.
+
+Some examples using react-helmet:
+* [Official GatsbyJS.org site](https://github.com/gatsbyjs/gatsby/blob/master/www/src/layouts/index.js)
+* [Jason Lengstorf's personal website](https://github.com/jlengstorf/lengstorf.com/blob/master/src/components/SEO.js)
+

www/src/pages/docs/doc-links.yaml

@@ -64,6 +64,8 @@
       link: /docs/plugin-authoring/
     - title: Proxying API Requests
       link: /docs/api-proxy/
+    - title: Search Engine Optimization (SEO)
+      link: /docs/seo/
     - title: Using CSS-in-JS Library Glamor
       link: /docs/glamor/
     - title: Using CSS-in-JS Library Styled Components