add top link to Mapping with Plot (#1175)

Author: Fil

README.md

@@ -8,6 +8,7 @@
 * [Transforms](https://observablehq.com/@observablehq/plot-transforms?collection=@observablehq/plot) - deriving data
 * [Facets](https://observablehq.com/@observablehq/plot-facets?collection=@observablehq/plot) - small multiples
 * [Legends](https://observablehq.com/@observablehq/plot-legends?collection=@observablehq/plot) - documenting visual encodings
+* [Mapping](https://observablehq.com/@observablehq/plot-mapping?collection=@observablehq/plot) - creating maps with geometries and projections
 
 This README is intended as a technical reference for Plot’s API. For more, please see: