minor #19110 Add info about feature freeze period (OskarStark)

This PR was squashed before being merged into the 6.3 branch.

Discussion
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Add info about feature freeze period

Follows
* symfony/symfony#52425 (comment)

Commits
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809f169 Add info about feature freeze period

Author: OskarStark

contributing/code/pull_requests.rst

@@ -147,6 +147,12 @@ work:
     for the ``5.4`` branch, the PR will also be applied by the core team on
     all the ``6.x`` branches that are still maintained.
 
+During the :ref:`stabilization phase <contributing-release-development>`, the development branch is in
+feature freeze. Please help the community prepare for the new version release. If you want to submit a
+new feature pull request, you should target the next version. For example, if ``6.3`` reached feature
+freeze, new features should target ``6.4``. If the ``6.4`` branch does not yet exist, target ``6.3``
+and rebase your pull requests once the branch is created.
+
 Create a Topic Branch
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