Update README.md

Author: jquense

README.md

@@ -12,6 +12,8 @@ It also allows "stacking" conditions via `when` for properties that depend on mo
 child property. Yup separates the parsing and validating functions into separate steps so it can be used to parse
 json separate from validating it, via the `cast` method.
 
+**Try it out:** https://runkit.com/jquense/yup#
+
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