doc: document task update-mods

Author: foosel

README.md

@@ -42,3 +42,7 @@ Converts the given single page to a page bundle. The single page's `content/<slu
 ### `task redate -- <path>`
 
 Change the given single page's or bundle's date to today.
+
+### `task update-mods`
+
+Updates the installed git submodules.

Taskfile.yml

@@ -124,5 +124,6 @@ tasks:
   update-mods:
     desc: Updates all submodules.
     cmds:
+      - git submodule update --remote --merge
       - cd themes/PaperMod && git pull origin master
       - cd themes/hugo-notice && git pull origin main