docs: add changelog

changelog/277.breaking.md

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+We removed the `sec_cats` entry from the metadata in a dataset's `attrs` in the native format
+as well as the interchange format. It did not add much value, but maintaining it was work, so on balance
+we decided to remove it.
+When reading datasets from disk (from the interchange format or netcdf files), `sec_cats` will be ignored
+so that datasets written with earlier versions of primap2 can still be read and produce valid in-memory
+datasets.