Adding exercise documentation

php/Exercise_Documentation.md

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+# Exercise Documentation
+
+This documentation serves the purpose to follow my train of thought in doing this exercise. 
+I will split the exercise into roughly 3 parts:
+
+- Add more tests to the exercise
+- refactor the code
+- implement the new feature
+
+
+## Tests
+
+we can add more tests using the criteria listed in the description of the exercise:
+
+- Once the sell by date has passed, Quality degrades twice as fast
+- The Quality of an item is never negative
+- “Aged Brie” actually increases in Quality the older it gets
+- The Quality of an item is never more than 50
+- “Sulfuras”, being a legendary item, never has to be sold or decreases in Quality
+- “Backstage passes”, like aged brie, increases in Quality as it’s SellIn value approaches; Quality increases by 2 when 
+- there are 10 days or less and by 3 when there are 5 days or less but Quality drops to 0 after the concert
+
+This will increase our chances we keep the original functionality working as is when refactoring. 
+
+## Refactor
+
+The plan is to refactor the updateQuality method and split out the above criteria for each different item. This will 
+make it easier to then implement an extended Item class to which we can delegate updating of the quality of the object.
+
+Because the updateQuality method is pretty cumbersome, it's probably easier to split it into smaller chunks first before
+creating separate classes for the objects.
+
+## Implement new feature
+
+Once we have finished with the, the new feature can be implemented in its own separate class, and for this and future 
+new items, we don't have to modify the core code anymore.