message: improve documentation for country maps
diff: @@ -1959,7 +1959,9 @@ List of Countries
  Need to add a new Country?
  -------------------------------
  -To add a new country in country map tools, we need to follow the following steps :
  +Warning: adding a new country is not easy and requires building superset from source!
  +
  +To add a new country in country map tools, you need to follow the following steps :
  1. You need shapefiles which contain data of your map.
  You can get this file on this site: https://www.diva-gis.org/gdata
  @@ -1970,13 +1972,17 @@ To add a new country in country map tools, we need to follow the following steps
  3. You need to convert shapefile to geojson file.
  This action can make with ogr2ogr tools: https://www.gdal.org/ogr2ogr.html
  -4. Put your geojson file in next folder : superset-frontend/src/visualizations/CountryMap/countries with the next name : nameofyourcountries.geojson
  +4. You can to reduce size of geojson file on this site: https://mapshaper.org/
  +
  +5. You will need to put your geojson file in the right place in the @superset-ui npm package. This is the "countries" folder in
  + ./superset-frontend/node_modules/@superset-ui/legacy-plugin-chart-country-map/esm/.
  + The .geojson files for other countries are already in this folder.
  -5. You can to reduce size of geojson file on this site: https://mapshaper.org/
  +6. You will also need to edit the "countries.js" file in the same directory, following the pattern of the other countries.
  -6. Go in file superset-frontend/src/explore/controls.jsx
  +7. Then go to the file superset-frontend/src/explore/controls.jsx
  -7. Add your country in component 'select_country'
  +8. Add your country in component 'select_country'
  Example :
  .. code-block:: javascript
  @@ -2005,3 +2011,6 @@ To add a new country in country map tools, we need to follow the following steps
  ].map(s => [s, s]),
  description: 'The name of country that Superset should display',
  },
  +
  +
  +9. Rebuild the front end from source and restart superset.
  