Add append (#579)

exercises/practice/leap/.docs/instructions.append.md

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+# Introduction append
+
+There are various ways to solve **leap**, this exercise focuses on not using the built-in `Time.leap_year?` method.
+This is because the goal of this exercise is to learn about dealing with comparisons and boolean logic.
+But for any other purpose, it is recommended to use the built-in method.