Added documentation for Knapsack

Competitive Coding/Greedy/Knapsack/README.md

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+# 0-1 Knapsack Problem
+-------------------------
+![](https://upload.wikimedia.org/wikipedia/commons/f/fd/Knapsack.svg)
+
+  The knapsack problem or **rucksack problem** is a problem in *combinatorial optimization* : Given a set of items, each with a weight and a value, determine the number of each item to include in a collection so that the total weight is less than or equal to a given limit and the total value is as large as possible. It derives its name from the problem faced by someone who is constrained by a fixed-size knapsack and must fill it with the most valuable items.
+
+  ## Applications
+  -----------------------------
+  Knapsack problems appear in real-world decision-making processes in a wide variety of fields:
+  * Finding the least wasteful way to cut raw materials
+  * Selection of investments and portfolios
+  * Selection of assets for asset-backed securitization
+  * Generating keys for the Merkle–Hellman cryptosystems
+
+## Computational Complexity
+  --------
+  * The decision problem form of the knapsack problem is NP-complete
+  * The optimization problem is NP-hard
+  * There is a pseudo-polynomial time algorithm using dynamic programming
+  * Many cases that arise in practice, and "random instances" from some distributions, can nonetheless be solved exactly
+
+  ![More info](https://en.wikipedia.org/wiki/Knapsack_problem)