update readme

README.md

@@ -57,4 +57,7 @@ The main change from the original container is the integration of jupyter-lab. W
 
 ``` 
 docker run -it -p 8888:8888 --mount type=bind,source="/path/to/target",target=/home s1tiling
-``` 
\ No newline at end of file
+``` 
+
+## SRTM data
+SRTM data must be provided with .hgt files. They are availables at Nasa Earth Explorer or here https://dwtkns.com/srtm30m/