Add info about feature freeze period

Author: OskarStark

contributing/code/pull_requests.rst

@@ -147,6 +147,15 @@ 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.
 
+.. warning::
+
+    For a short timeframe before a new major version is released, we are in a
+    feature freeze period, this means, your new feature PRs must target the next
+    minor version of the next major version. For example, if the current major
+    version is ``6.4``, you should target ``7.1``. If the ``7.1`` branch is not
+    created yet, target the most recent one, in this case ``7.0`` and rebase your
+    PR later, once the ``7.1`` branch is created.
+
 Create a Topic Branch
 ~~~~~~~~~~~~~~~~~~~~~