add mdc migration guide to guide list (#1176)

Author: mmalerba

src/app/shared/guide-items/guide-items.ts

@@ -73,6 +73,12 @@ const GUIDES = [
     name: 'Duplicate theming styles',
     document: '/docs-content/guides/duplicate-theming-styles.html',
     overview: 'Learn about our new color mixins for Sass that avoid duplicating theming styles.'
+  },
+  {
+    id: 'mdc-migration',
+    name: 'Migrating to MDC-based Components',
+    document: '/docs-content/guides/v15-mdc-migration.html',
+    overview: 'Learn how to migrate your app to the new MDC-based components available in v15.'
   }
 ];