Add a story to the flatten-array exercise (#2538)

Author: tasxatzial

exercises/flatten-array/description.md renamed to exercises/flatten-array/instructions.md

@@ -1,4 +1,4 @@
-# Description
+# Instructions
 
 Take a nested array of any depth and return a fully flattened array.
 

exercises/flatten-array/introduction.md

@@ -0,0 +1,7 @@
+# Introduction
+
+A shipment of emergency supplies has arrived, but there's a problem.
+To protect from damage, the items — flashlights, first-aid kits, blankets — are packed inside boxes, and some of those boxes are nested several layers deep inside other boxes!
+
+To be prepared for an emergency, everything must be easily accessible in one box.
+Can you unpack all the supplies and place them into a single box, so they're ready when needed most?