Add integer partition.

src/algorithms/math/integer-partition/README.md

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+# Integer Partition
+
+In number theory and combinatorics, a partition of a positive 
+integer `n`, also called an **integer partition**, is a way of 
+writing `n` as a sum of positive integers. 
+
+Two sums that differ only in the order of their summands are 
+considered the same partition. For example, `4` can be partitioned 
+in five distinct ways:
+
+```
+4
+3 + 1
+2 + 2
+2 + 1 + 1
+1 + 1 + 1 + 1
+```
+
+The order-dependent composition `1 + 3` is the same partition
+as `3 + 1`, while the two distinct 
+compositions `1 + 2 + 1` and `1 + 1 + 2` represent the same 
+partition `2 + 1 + 1`.
+
+Young diagrams associated to the partitions of the positive
+integers `1` through `8`. They are arranged so that images 
+under the reflection about the main diagonal of the square 
+are conjugate partitions.
+
+![Integer Partition](https://upload.wikimedia.org/wikipedia/commons/d/d8/Ferrer_partitioning_diagrams.svg)
+
+## References
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Partition_(number_theory))