docs: more info in warning

Author: gadomski

README.md

@@ -3,6 +3,7 @@
 > [!WARNING]
 > This repo does not have an active maintainer.
 > If you'd like to help with maintenance, please open an [issue](https://github.com/stac-utils/stactools/issues) saying as much.
+> For more background, see [this discussion](https://github.com/radiantearth/stac-spec/discussions/1331).
 
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