Add link to new docs file about --filter

Author: weitzman

mkdocs.yml

@@ -14,6 +14,7 @@ pages:
   - Cron: cron.md
   - Drush configuration: using-drush-configuration.md
   - Drupal configuration: config-exporting.md
+  - Output Formats, Fields & Filters: output-formats-filters.md
   - REPL (a custom shell for Drupal): repl.md
 - Writing commands/generators:
   - Command Authoring: commands.md