Add Hamming.

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

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+# Hamming Distance
+
+the Hamming distance between two strings of equal length is the 
+number of positions at which the corresponding symbols are 
+different. In other words, it measures the minimum number of
+substitutions required to change one string into the other, or 
+the minimum number of errors that could have transformed one 
+string into the other. In a more general context, the Hamming 
+distance is one of several string metrics for measuring the 
+edit distance between two sequences.
+
+## Examples
+
+The Hamming distance between:
+
+- "ka**rol**in" and "ka**thr**in" is **3**.
+- "k**a**r**ol**in" and "k**e**r**st**in" is **3**.
+- 10**1**1**1**01 and 10**0**1**0**01 is **2**.
+- 2**17**3**8**96 and 2**23**3**7**96 is **3**.
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Hamming_distance)