Add Levenshtein.

src/algorithms/string/levenshtein-distance/README.md

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+# Levenshtein Distance
+
+The Levenshtein distance is a string metric for measuring the 
+difference between two sequences. Informally, the Levenshtein 
+distance between two words is the minimum number of 
+single-character edits (insertions, deletions or substitutions) 
+required to change one word into the other.
+
+## Definition
+
+Mathematically, the Levenshtein distance between two strings
+`a` and `b` (of length `|a|` and `|b|` respectively) is given by
+![Levenshtein](https://wikimedia.org/api/rest_v1/media/math/render/svg/4cf357d8f2135035207088d2c7b890fb4b64e410)
+where
+
+![Levenshtein](https://wikimedia.org/api/rest_v1/media/math/render/svg/f0a48ecfc9852c042382fdc33c19e11a16948e85)
+
+where 
+![Levenshtein](https://wikimedia.org/api/rest_v1/media/math/render/svg/52512ede08444b13838c570ba4a3fc71d54dbce9)
+is the indicator function equal to `0` when
+![Levenshtein](https://wikimedia.org/api/rest_v1/media/math/render/svg/231fda9ee578f0328c5ca28088d01928bb0aaaec)
+and equal to 1 otherwise, and
+![Levenshtein](https://wikimedia.org/api/rest_v1/media/math/render/svg/bdc0315678caad28648aafedb6ebafb16bd1655c)
+is the distance between the first `i` characters of `a` and the first 
+`j` characters of `b`.
+
+Note that the first element in the minimum corresponds to 
+deletion (from `a` to `b`), the second to insertion and 
+the third to match or mismatch, depending on whether the 
+respective symbols are the same.
+
+## Example
+
+For example, the Levenshtein distance between `kitten` and 
+`sitting` is `3`, since the following three edits change one 
+into the other, and there is no way to do it with fewer than 
+three edits:
+
+1. **k**itten → **s**itten (substitution of "s" for "k")
+2. sitt**e**n → sitt**i**n (substitution of "i" for "e")
+3. sittin → sittin**g** (insertion of "g" at the end).
+
+## References
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Levenshtein_distance)