Update READMEs.

Author: trekhleb

src/data-structures/tree/avl-tree/README.md

@@ -44,3 +44,4 @@ AVL tree with balance factors (green)
 * [Wikipedia](https://en.wikipedia.org/wiki/AVL_tree)
 * [Tutorials Point](https://www.tutorialspoint.com/data_structures_algorithms/avl_tree_algorithm.htm)
 * [BTech](http://btechsmartclass.com/DS/U5_T2.html)
+* [AVL Tree Insertion on YouTube](https://www.youtube.com/watch?v=rbg7Qf8GkQ4&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8&index=12&)

src/data-structures/tree/binary-search-tree/README.md

@@ -29,4 +29,5 @@ The leaves are not drawn.
 
 ## References
 
-[Wikipedia](https://en.wikipedia.org/wiki/Binary_search_tree)
+- [Wikipedia](https://en.wikipedia.org/wiki/Binary_search_tree)
+- [Inserting to BST on YouTube](https://www.youtube.com/watch?v=wcIRPqTR3Kc&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8&index=9&t=0s)