docs: Add release note for bootupd support

docs/release-notes/bootupd.rst

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+:Type: OSTree
+:Summary: Preliminary support for bootable ostree containers
+
+:Description:
+    Anaconda can now correctly detect and use the bootupd bootloader used in
+    bootable ostree containers. When the installed container includes the ``bootupctl`` tool, it
+    is used instead of installing the ``grub2`` bootloader by Anaconda.
+
+    At this point, a matching partitioning must still be manually set up in kickstart, together
+    with the ``ostreecontainer`` command.
+
+:Links:
+    - https://github.com/rhinstaller/anaconda/pull/5342