Update ERRORS description

ERRORS.md

@@ -58,6 +58,11 @@ Validata extends frictionless, bringing additional checks (located in [custom-ch
 
   - emit `nomenclature-actes-value` error
 
+- `OpeningHoursValueError` checks that a value is a valid opening hours
+  (see [spec](https://wiki.openstreetmap.org/wiki/Key:opening_hours))
+
+  - emit `opening-hours-value` error
+
 - `SumColumnsValue` checks that a field contains the sum of some other fields
 
   - emit `sum-columns-value` error