DOC: Add leaflet-providers links

Author: martinfleis

README.md

@@ -74,10 +74,19 @@ After the installation, you will find the JSON used as a database of providers i
 [`geopandas`](https://github.com/geopandas) GitHub organization. You can see a full list
 of contributors [here](https://github.com/geopandas/xyzservices/graphs/contributors).
 
+The main group of providers is retrieved from the [`leaflet-providers`
+project](https://github.com/leaflet-extras/leaflet-providers) that contains both openly
+accessible providers as well as those requiring registration. All of them are considered
+[free](https://github.com/leaflet-extras/leaflet-providers/blob/master/README.md#what-do-we-mean-by-free).
+
 If you would like to contribute to the project, have a look at the list of
 [open issues](https://github.com/geopandas/contextily/issues), particularly those labeled as
 [good first issue](https://github.com/geopandas/xyzservices/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22).
 
 ## License
 
 BSD 3-Clause License
+
+Resources coming from the [`leaflet-providers`
+project](https://github.com/leaflet-extras/leaflet-providers) are licensed under BSD
+2-Clause License (© 2013 Leaflet Providers)