Added soil type classification in GIS documentation.

docs/src/gis.md

@@ -104,11 +104,35 @@ Then you need to align and crop it to the extent you require, using the dialog a
 `Raster -> Align Rasters...`. Select your landcover map as the reference layer
 and the extent layer, then choose your national soil map as the input. (Don't forget
 to define the output file name using `Configure Raster...`, this is a bit hidden.)
-The created output file can then be used for the `soilmap` parameter.
+The created output file can then be used for the `soilmap` parameter. Its integer
+values map onto the `SoilType` enum as follows:
+
+```
+1: Abbauflächen -> nosoil
+2: Gewässer -> nosoil
+3: Lehmsande (ls) -> loamy_sand
+4: Lehmschluffe (lu) -> silt_loam
+5: Moore -> nosoil
+6: Normallehme (ll) -> loam
+7: Reinsande (ss) -> sand
+8: Sandlehme (sl) -> sandy_loam
+9: Schluffsande (us) -> sandy_loam
+10: Schlufftone (ut) -> silty_clay
+11: Siedlung -> nosoil
+12: Tonlehme (tl) -> clay_loam
+13: Tonschluffe (tu) -> silty_clay_loam
+14: Watt -> nosoil
+```
+
+![Soil types triangle](assets/soil_types_english.png)
+
+*Names of soil types are based on the relative composition of clay, silt, and sand.
+Note that the typology used in the Bodenatlas does not map perfectly on to this
+international classification. Image source: [Australian Environmental Education](https://www.australianenvironmentaleducation.com.au/education-resources/what-is-soil/)*
 
 ## Weather data
 
-Currently, Persefone uses historical weather data from the closes weather station
+Currently, Persefone uses historical weather data from the closest weather station
 as its weather input. (In future, this may be changed to a more detailed raster
 input, which could then also provide future weather predictions under climate change.)
 Weather data can be downloaded from the [German weather service (DWD)](https://www.dwd.de/DE/leistungen/cdc/cdc_ueberblick-klimadaten.html?nn=16102).