Update READMEs.

Author: trekhleb

src/data-structures/graph/README.md

@@ -20,4 +20,6 @@ represented by integer indices or references.
 
 ## References
 
-[Wikipedia](https://en.wikipedia.org/wiki/Graph_(abstract_data_type))
+- [Wikipedia](https://en.wikipedia.org/wiki/Graph_(abstract_data_type))
+- [Introduction to Graphs on YouTube](https://www.youtube.com/watch?v=gXgEDyodOJU&index=9&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8)
+- [Graphs representation on YouTube](https://www.youtube.com/watch?v=k1wraWzqtvQ&index=10&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8)