docs: updating the project (#520)

* doc: add template-sync tool to the documentation

* docs: add a  link tu update.md in README

* fix: pr review

* Update README.md

* Update and rename update.md to updating.md

---------

Co-authored-by: Kévin Dunglas <kevin@dunglas.fr>

docs/updating.md

@@ -0,0 +1,15 @@
+# Updating Your Project
+
+To import the changes made to the *Symfony Docker* template into your project, we recommend using
+[*template-sync*](https://github.com/mano-lis/template-sync):
+
+1. Run the script to synchronize your project with the latest version of the skeleton:
+
+    ```console
+    curl -sSL https://raw.githubusercontent.com/mano-lis/template-sync/main/template-sync.sh | sh -s -- https://github.com/dunglas/symfony-docker
+    ```
+
+2. Resolve conflicts, if any
+3. Run `git cherry-pick --continue`
+
+For more advanced options, refer to [the documentation of *template sync*](https://github.com/coopTilleuls/template-sync#template-sync).