data: add US census data to the atlas

data/atlas/README.md

@@ -24,5 +24,10 @@ The atlas dataset contains data about places in the world from a variety of sour
 <td><code>worldmap</code></td>
 <td>World</td>
 </tr>
+<tr>
+<td><a href="https://www.census.gov/data/datasets/time-series/demo/popest/2010s-counties-total.html#par_textimage_70769902">US Census</a></td>
+<td><code>us_census</code></td>
+<td>USA</td>
+</tr>
 </tbody>
 </table>