[Leap] Make message to ignore built-in library method C# specific (#612)

* Make message to ignore built-in library method C# specific

* Revert "Make message to ignore built-in library method C# specific"

This reverts commit df5d048.

* Add link to built in IsLeapYear function in hints.md

* Integrate PR feedback

Author: robkeim

exercises/leap/.meta/hints.md

@@ -0,0 +1,4 @@
+## Notes
+
+The DateTime class in C# provides a built-in [IsLeapYear](https://msdn.microsoft.com/en-us/library/system.datetime.isleapyear(v=vs.110).aspx) method
+which you should pretend doesn't exist for the purposes of implementing this exercise.