reference to companion command-line utility

Author: gorhill

README.md

@@ -6,6 +6,8 @@ In another project, I decided to use cron expression syntax to encode scheduling
 
 The time-matching algorithm in this implementation is efficient, it avoids as much as possible to guess the next matching time stamp, a common technique seen in a number of implementations out there.
 
+There is also a companion command-line utility to evaluate cron time expressions: <https://github.com/gorhill/cronexprdo>.
+
 Implementation
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 The reference documentation for this implementation is found at