Add short Big O explanation to the main README.

Author: trekhleb

README.md

@@ -242,7 +242,6 @@ npm test -- 'playground'
 ### Big O Notation
 
 *Big O notation* is used to classify algorithms according to how their running time or space requirements grow as the input size grows.
-
 On the chart below you may find most common orders of growth of algorithms specified in Big O notation.
 
 ![Big O graphs](./assets/big-o-graph.png)