Add the changelog entry for weather product display metadata

changelog.d/1494-weather-product-display.md

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+- Let a weather source describe its own products through an optional
+  `display` object in each manifest (source, credit, units, coverage, info,
+  legend and extent). Each valid field replaces the built-in value, and
+  manifests without it render as before (samehkhamis, #1494).