Add LCS.

src/algorithms/string/longest-common-substring/README.md

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+# Longest Common Substring Problem
+
+The longest common substring problem is to find the longest string 
+(or strings) that is a substring (or are substrings) of two or more 
+strings.
+
+## Example
+
+The longest common substring of the strings `ABABC`, `BABCA` and 
+`ABCBA` is string `ABC` of length 3. Other common substrings are
+`A`, `AB`, `B`, `BA`, `BC` and `C`.
+
+```
+ABABC
+  |||
+ BABCA
+  |||
+  ABCBA
+```
+
+## References
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Longest_common_substring_problem)
+- [YouTube](https://www.youtube.com/watch?v=BysNXJHzCEs)