Add Justify as a suite user to README.md

Author: leadpony

README.md

@@ -100,6 +100,7 @@ This suite is being used by:
 * [json-schema-validator](https://github.com/daveclayton/json-schema-validator)
 * [everit-org/json-schema](https://github.com/everit-org/json-schema)
 * [networknt/json-schema-validator](https://github.com/networknt/json-schema-validator)
+* [Justify](https://github.com/leadpony/justify)
 
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