data: added README for atlas

data/atlas/README.md

+# Atlas
+
+The atlas dataset contains data about places in the world from a variety of sources.
+
+
+## Sources
+
+<table class="table">
+<thead>
+<tr>
+<th>Source</th>
+<th>Folder</th>
+<th>Region</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td><a href="https://data.worldbank.org/indicator/SP.POP.TOTL">Worldbank</a></td>
+<td><code>worldbank</code></td>
+<td>World</td>
+</tr>
+<tr>
+<td><a href="https://worldmap.harvard.edu/data/geonode:country_centroids_az8">Harvard Worldmap</a></td>
+<td><code>worldmap</code></td>
+<td>World</td>
+</tr>
+</tbody>
+</table>