feat(graduated-tiered-pricing): add specification

Author: santakadev

exercises/graduated_tiered_prices/README.md

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+# Graduated Tiered pricing
+
+A more advanced variation of the [Tiered Pricing](../tiered_pricing/README.md) exercise.
+
+## User story
+
+Companies want to get Codely subscriptions for their
+employees.
+
+We need to calculate the total price of the subscription
+given the number of subscription that a company
+wants to purchase.
+
+Each subscription has a different unit price, depending
+on the tier it belongs to:
+
+| Range | Unit price |
+|------:|-----------:|
+|   1-2 |      299 € |
+|  3-10 |      239 € |
+| 11-25 |      219 € |
+| 26-50 |      199 € |
+|   51+ |      149 € |
+
+For example, if the company wants 3 subscription,
+the price of each subscription would be:
+
+| subscription # | Price |
+|---------------:|------:|
+|              1 | 299 € |
+|              2 | 299 € |
+|              3 | 239 € |
+|      **Total** | 837 € |
+
+Here are some examples that can guide you during the
+development process:
+
+| Subscriptions | Unit price | Total   |
+|--------------:|-----------:|--------:|
+| 1             |      299 € |   299 € |
+| 2             |      299 € |   598 € |
+| 3             |      239 € |   837 € |
+| 4             |      239 € |  1076 € |
+| 5             |      239 € |  1315 € |
+| 11            |      219 € |  2729 € |
+| 12            |      219 € |  2948 € |
+| 13            |      219 € |  3167 € |
+| 26            |      199 € |  5994 € |
+| 27            |      199 € |  6193 € |
+| 28            |      199 € |  6392 € |
+| 50            |      149 € | 10770 € |
+| 51            |      149 € | 10919 € |
+| 52            |      149 € | 11068 € |