Improvement docu: Legend of TAXNWRB values

[A-Mue]

Maybe, this could be added to the SoilDataPrep documentation - some meta data on the TAXNWRB soil classes:

SoilGrids IDs and classes:

• Codes and names:

ftp://ftp.soilgrids.org/legends/TAXNWRB.qml
(explanation: SoilDataPrep creates a tif-map called "soils_catchment", with the most probable soil class; only values are assigned to each pixel. Here you can find, which value is which FAO soil class.)

• Legend & short characteristic of soil types:

ftp://ftp.soilgrids.org/data/recent/SoilGrids250m_TAXNWRB_legend_June_2016.pdf

• Index of all SoilGrids legends:

ftp://ftp.soilgrids.org/legends/

Explanation & paper on SoilGrids:

• General explanation SoilGrids:
  https://www.isric.org/explore/soilgrids/
• SoilGrids250m:
  Global gridded soil information based on machine learning:
  https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0169748#pone.0169748.ref015

FAO legend & explanation of soil types:

• World reference base for soil resources 2014:
  http://www.fao.org/3/i3794en/I3794en.pdf