Update READMEs.

Author: trekhleb

src/algorithms/math/power-set/README.md

@@ -3,3 +3,10 @@
 Power set of a set A is the set of all of the subsets of A.
 
 Eg. for `{x, y, z}`, the subsets are : `{{}, {x}, {y}, {z}, {x, y}, {x, z}, {y, z}, {x, y, z}}`
+
+![Power Set](https://www.mathsisfun.com/sets/images/power-set.svg)
+
+## References
+
+* [Wikipedia](https://en.wikipedia.org/wiki/Power_set)
+* [Math is Fun](https://www.mathsisfun.com/sets/power-set.html)