give example for LEAFLET_TILESERVER_ATTRIBUTION

README.md

@@ -70,6 +70,7 @@ Example:
 ```
 LEAFLET_TILESERVER_URL=https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png
 LEAFLET_TILESERVER_SUBDOMAINS=abc
+LEAFLET_TILESERVER_ATTRIBUTION="&copy; <a href=\"https://www.openstreetmap.org/copyright\">OpenStreetMap</a> contributors"
 ```
 
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