Extend description of test suite

test/suite/README.md

@@ -20,6 +20,15 @@ Each test is an object with keys:
   * `occurrence`(optional)
   * either `value` or `subfields`
 
-* `errors` an array of expected errors. If no errors are given, the validation is expected to pass.
+* `errors` an array of expected errors. If no errors are given, the validation is expected to pass. Each error can have keys:
+
+  * `error message` human readable error message 
+  * `error` error code, corresponding to the [rule number from Avram specification](https://format.gbv.de/schema/avram/specification#validation-rules) (e.g. `AR2` for an unknown field)
+  * `code` subfield code an error was found in
+  * `tag` tag of the field an error was found in
+  * `occurrence` occurrence of the field an error was found in
+  * `identifier` field identifier from the field schedule for the field an error was found in
+  * `pattern` regular expression that was violated
+  * `value` malformed or disallowed value that caused an error
 
 * `options` an optional array of validation options, each given as string. The values are added to options given for the full test file, of available.