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Add knapsack problem.

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Oleksii Trekhleb
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src/algorithms/sets/knapsack-problem/README.md
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@ -17,6 +17,52 @@ while still keeping the overall weight under or equal to 15 kg?
![knapsack problem](https://upload.wikimedia.org/wikipedia/commons/f/fd/Knapsack.svg)
## Definition
### 0/1 knapsack problem
The most common problem being solved is the **0/1 knapsack problem**,
which restricts the number `xi` of copies of each kind of item to zero or one.
Given a set of n items numbered from `1` up to `n`, each with a
weight `wi` and a value `vi`, along with a maximum weight
capacity `W`,
maximize ![0/1 knapsack](https://wikimedia.org/api/rest_v1/media/math/render/svg/85620037d368d2136fb3361702df6a489416931b)
subject to ![0/1 knapsack](https://wikimedia.org/api/rest_v1/media/math/render/svg/dd6e7c9bca4397980976ea6d19237500ce3b8176)
and ![0/1 knapsack](https://wikimedia.org/api/rest_v1/media/math/render/svg/07dda71da2a630762c7b21b51ea54f86f422f951)
Here `xi` represents the number of instances of item `i` to
include in the knapsack. Informally, the problem is to maximize
the sum of the values of the items in the knapsack so that the
sum of the weights is less than or equal to the knapsack's
capacity.
### Bounded knapsack problem (BKP)
The **bounded knapsack problem (BKP)** removes the restriction
that there is only one of each item, but restricts the number
`xi` of copies of each kind of item to a maximum non-negative
integer value `c`:
maximize ![bounded knapsack](https://wikimedia.org/api/rest_v1/media/math/render/svg/85620037d368d2136fb3361702df6a489416931b)
subject to ![bounded knapsack](https://wikimedia.org/api/rest_v1/media/math/render/svg/dd6e7c9bca4397980976ea6d19237500ce3b8176)
and ![bounded knapsack](https://wikimedia.org/api/rest_v1/media/math/render/svg/6c8c5ac4f8247b3b8e01e89de76a1df0ea969821)
### Unbounded knapsack problem (UKP)
The **unbounded knapsack problem (UKP)** places no upper bound
on the number of copies of each kind of item and can be
formulated as above except for that the only restriction
on `xi` is that it is a non-negative integer.
maximize ![unbounded knapsack](https://wikimedia.org/api/rest_v1/media/math/render/svg/85620037d368d2136fb3361702df6a489416931b)
subject to ![unbounded knapsack](https://wikimedia.org/api/rest_v1/media/math/render/svg/dd6e7c9bca4397980976ea6d19237500ce3b8176)
and ![unbounded knapsack](https://wikimedia.org/api/rest_v1/media/math/render/svg/90a99710f61d5dea19e49ae5b31164d2b56b07e3)
## References
- [Wikipedia](https://en.wikipedia.org/wiki/Knapsack_problem)