;dev: note bitrotted error message tests

hledger/test/errors/README.md

@@ -21,6 +21,9 @@ Each error has
 - sometimes supplementary data files (like `csvdateparse.csv.rules`)
 - an autogenerated shelltestrunner test (like `balanced.test`)
 
+*XXX: This is hard to maintain, and has bitrotted/had manual fixups;
+the following won't generate accurate passing tests exactly*
+
 To update this readme with the latest error messages, run `make readme`.
 To regenerate the shelltestrunner tests, run `make tests`.
 To test all of these error messages with hledger, run `make test`.