docs/news: describe new blueprint kernel option

docs/news/unreleased/kernel-customizations-name.md

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+# Blueprint: Kernel name customization
+
+When creating ostree commits, only one kernel package can be installed at a
+time, otherwise creating the commit will fail in rpm-ostree.  This prevents
+ostree type builds (RHEL for Edge and Fedora IoT) to add alternative kernels,
+in particular, the real-time kernel (`kernel-rt`).
+
+Blueprints now support defining the name of the kernel to be used in an image,
+through the `customizations.kernel.name` key.  If not specified, the default
+`kernel` package is included as before.
+
+Relevant PRs:
+https://github.com/osbuild/osbuild-composer/pull/1175