README: add a helpful note for people who _already_ cloned

The advice to use Scalar comes too late for contributors who already
have obtained a (full) clone. To help them, let's just suggest to
convert the checkout into a sparse one.

Suggested-by: Toon Claes <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

README.md

@@ -24,6 +24,8 @@ $ cd git-scm.com
 $ git sparse-checkout set layouts content static assets hugo.yml data script
 $ git reset --hard
 ```
+> [!NOTE]
+> If you _already_ have a full clone and wish to accelerate development by focusing only on a small subset of the pages, you may want to run the `git sparse-checkout set [...]` command mentioned above.
 
 Here is a detailed list of the relevant directories: