chore: add github icon

Author: jpedroschmitz

src/components/footer.js

@@ -6,6 +6,11 @@ import SlackSVG from '../svg/slack.svg'
 import TwitterSVG from '../svg/twitter.svg'
 
 const socialLinks = [
+  {
+    Component: GitHubSVG,
+    href: 'https://github.com/graphcms/gatsby-graphcms-ecommerce-starter',
+    title: 'GitHub',
+  },
   {
     Component: SlackSVG,
     href: 'http://slack.graphcms.com',