Update CHANGELOG

CHANGELOG.md

 # CHANGELOG
 
+## next
+
+- Each custom check now gets a unique code
+- Custom check now emits `check-error` errors during `validate-start` method
+- Unknown custom check in a schema emits a `check-error`
+
 ## 0.8.3
 
 - Really fix previous xlsx issue (for uploaded file AND url resource)