readme: document topological sort

Author: mpartel

README.md

@@ -1,16 +1,39 @@
 # Xi Algorithm
 
+A collection of miscellaneous algorithms.
+
 ## Luhn
 
 Usage:
 
-```
+```php
 use Xi\Algorithm\Luhn;
 
 $luhn = new Luhn();
 $luhn->generate(123); // 1230
 ```
 
+## Topological sort
+
+Sorts the nodes of an acyclic graph so that if node X points to node Y then Y appears before X in the list. [Read more.](http://en.wikipedia.org/wiki/Topological_sorting)
+
+Basically, it's useful for resolving a dependency graph.
+
+Usage:
+
+```php
+// A description of a graph:
+$edges = array(
+    'B' => array('C', 'D'),   // Node B points to nodes C and D
+    'A' => array('B'),        // Node A points to node B
+    'C' => array('D'),        // Node C points to node D
+);
+
+$nodesSorted = \Xi\Algorithm\TopologicalSort::apply($edges);
+// $nodesSorted is now array('D', 'C', 'B', 'A')
+```
+
+
 ## Running the tests
 
 No dependencies to other libraries exist, but in order to generate an autoloader