Improve docs #1

Author: m-mohr

README.md

@@ -9,7 +9,8 @@
 This document explains the Administrative Division Extension to the
 [Field Boundaries for Agriculture (fiboa) Specification](https://github.com/fiboa/specification).
 
-It defines administrative divisions on the country and subdivision level based on ISO-3166.
+It defines administrative divisions on the country and subdivision level based on
+[ISO-3166](https://www.iso.org/iso-3166-country-codes.html).
 
 - Examples:
   - [GeoJSON](examples/geojson/)
@@ -26,8 +27,22 @@ The properties in the table below can be used in these parts of fiboa documents:
 
 | Property Name          | Type   | Description |
 | ---------------------- | ------ | ----------- |
-| admin:country_code     | string | **REQUIRED.** ISO 3166-1 alpha-2 country code (aka admin0). Two-letter country code for the country that contains the field, e.g. DE for Germany. Can be found at <https://www.iso.org/obp/ui/#search> under the Alpha-2 code column. |
-| admin:subdivision_code | string | ISO 3166-2 codes for identifying the principal subdivisions (e.g., provinces or states) of a country (aka admin1) that contains the field. The feature only contains the second part of the ISO 3166-2 code, to reduce redundancy. |
+| admin:country_code     | string | **REQUIRED.** ISO 3166-1 alpha-2 country code (aka admin0). Two-letter country code for the country that contains the field. |
+| admin:subdivision_code | string | ISO 3166-2 codes for identifying the principal subdivisions (e.g., provinces or states) of a country (aka admin1) that contains the field. The feature only contains the second part of the ISO 3166-2 code to reduce redundancy. |
+
+The codes can be found at:
+
+- Search for country codes at <https://www.iso.org/obp/ui/#search> via the Alpha-2 code column.
+- The subdivision codes can then be accessed at <https://www.iso.org/obp/ui/#iso:pub:PUB500001:en>
+
+Please note that for some countries subdivision codes are not available.
+
+### Examples
+
+1. California, USA:
+   `admin:country_code` = `US` and `admin:subdivision_code` = `CA`
+2. Bavaria, Germany:
+   `admin:country_code` = `DE` and `admin:subdivision_code` = `BY`
 
 ## Contributing