Add article to SUMMARY.md

Author: kjac

15/umbraco-cms/SUMMARY.md

@@ -27,6 +27,7 @@
     * [Version Specific Upgrades](fundamentals/setup/upgrading/version-specific/README.md)
       * [Upgrade from Umbraco 8 to the latest version](fundamentals/setup/upgrading/version-specific/upgrade-from-8-to-latest.md)
       * [Migrate content to Umbraco 15](fundamentals/setup/upgrading/version-specific/migrate-content-to-umbraco-15.md)
+      * [Migrate custom Property Editors to Umbraco 14+](fundamentals/setup/upgrading/version-specific/migrate-custom-property-editors-to-umbraco-14.md)
       * [Migrate content to Umbraco 8](fundamentals/setup/upgrading/version-specific/migrate-content-to-umbraco-8.md)
       * [Minor upgrades for Umbraco 8](fundamentals/setup/upgrading/version-specific/minor-upgrades-for-umbraco-8.md)
       * [Upgrade to Umbraco 7](fundamentals/setup/upgrading/version-specific/upgrade-to-umbraco-7.md)