fixed subtree commands for release pipeline

CONTRIBUTING.md

@@ -11,10 +11,16 @@ Check out [Typst packages repo](https://github.com/typst/packages) to learn more
 I personally suggest the following steps :
 
 - fork the Typst packages repo
-- in `packages/preview/polytechnique-reports`, for every `x.y.z` version (if the `main` branch currently contains the `x.y.z` release):
+- in the root directory, for every `x.y.z` version (if the `main` branch currently contains the `x.y.z` release) :
 
     ```bash
-    git subtree pull --prefix=x.y.z [email protected]:remigerme/typst-polytechnique.git main
+    git subtree add --prefix=packages/preview/polytechnique-reports/x.y.z [email protected]:remigerme/typst-polytechnique.git main
+    ```
+
+    You may need to pull sometimes (if the release wasn't fully ready) :
+
+    ```bash
+    git subtree pull --prefix=packages/preview/polytechnique-reports/x.y.z [email protected]:remigerme/typst-polytechnique.git main
     ```
 
 - PR your fork to publish the package