Update READMEs.

trekhleb · trekhleb · commit d3d8a467e540 · 2018-04-13T17:38:00.000+03:00
diff --git a/src/algorithms/string/permutations/README.md b/src/algorithms/string/permutations/README.md
@@ -7,3 +7,13 @@ A string of length `n` has `n!` permutation.
 Below are the permutations of string `ABC`.
 
 `ABC ACB BAC BCA CBA CAB`
+
+When the order doesn't matter, it is a **Combination**.
+
+When the order **does** matter it is a **Permutation**.
+
+![Permutation Lock](https://www.mathsisfun.com/combinatorics/images/combination-lock.jpg)
+
+## References
+
+[Math Is Fun](https://www.mathsisfun.com/combinatorics/combinations-permutations.html)
