docs: add rhel85 filesystem support release note

docs/news/unreleased/rhel8.5-filesystem-support.md

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+# Add custom file system support for RHEL 8.5
+
+The `weldr` api has been extended to support custom file systems for RHEL 8.5.
+Filesystem `mountpoints` and minimum partition `size` can be set under blueprint customizations, as below:
+
+```toml
+[[customizations.filesystem]]
+mountpoint = "/"
+size = 2147483648
+```
+
+The following `mountpoints` are supported:
+
+- `/var`
+- `/var/*`
+- `/home`
+- `/opt`
+- `/srv`
+- `/usr`
+- `/`