add aeronet sda reader minimal documentation

README.md

@@ -7,12 +7,19 @@ implementations of readers for the pyaerocom project using pyaro as interface
 This will install pyaro and pyaro-readers and all their dependencies.
 
 ## Supported readers
-* aeronetsunreader  
-Reader for aeronet sun version 3 data.  
+### aeronetsunreader  
+Reader for aeronet sun version 3 data (https://aeronet.gsfc.nasa.gov/new_web/download_all_v3_aod.html).  
 The reader supports reading from an uncompressed local file and from an URL providing a zip file or an
 uncompressed file.  
 If a zip file URL is provided, only the 1st file in there is used (since the 
-Aeronet provided zip contains all data in a single file)
+Aeronet provided zip contains all data in a single file).
+
+### aeronetsdareader
+Reader for aeronet SDA version 3 data (https://aeronet.gsfc.nasa.gov/new_web/download_all_v3_aod.html).  
+The reader supports reading from an uncompressed local file and from an URL providing a zip file, an
+uncompressed file or a tar file (including all common compression formats).  
+If a zip file URL is provided, only the 1st file in there is used (since the 
+Aeronet provided zip contains all data in a single file).
 
 ## Usage
 ### aeronetsunreader
@@ -38,6 +45,30 @@ ts.data('AOD_550nm')['altitudes']
 ts.data('AOD_550nm')['values']
 
 ```
+### aeronetsdareader
+```python
+import pyaro.timeseries
+TEST_URL = "https://pyaerocom.met.no/pyaro-suppl/testdata/SDA_Level20_Daily_V3_testdata.tar.gz"
+engine = pyaro.list_timeseries_engines()["aeronetsdareader"]
+ts = engine.open(TEST_URL, filters=[], fill_country_flag=False)
+print(ts.variables())
+# stations
+ts.data('AODGT1_550nm')['stations']
+# start_times
+ts.data('AODGT1_550nm')['start_times']
+# stop_times
+ts.data('AODGT1_550nm')['stop_times']
+# latitudes
+ts.data('AODGT1_550nm')['latitudes']
+# longitudes
+ts.data('AODGT1_550nm')['longitudes']
+# altitudes
+ts.data('AODGT1_550nm')['altitudes']
+# values
+ts.data('AODGT1_550nm')['values']
+
+```
+
 ### geocoder_reverse_natural_earth
 geocoder_reverse_natural_earth is small helper to identify country codes for obs networks that don't mention the
 countrycode of a station in their location data