Add README.

Author: trekhleb

src/data-structures/tree/README.md

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+# Tree
+
+* [Binary Search Tree](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/tree/binary-search-tree)
+* [AVL Tree](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/tree/avl-tree)
+
+In computer science, a tree is a widely used abstract data 
+type (ADT) — or data structure implementing this ADT—that 
+simulates a hierarchical tree structure, with a root value 
+and subtrees of children with a parent node, represented as 
+a set of linked nodes.
+
+A tree data structure can be defined recursively (locally) 
+as a collection of nodes (starting at a root node), where 
+each node is a data structure consisting of a value, 
+together with a list of references to nodes (the "children"), 
+with the constraints that no reference is duplicated, and none 
+points to the root.
+
+A simple unordered tree; in this diagram, the node labeled 7 has
+two children, labeled 2 and 6, and one parent, labeled 2. The
+root node, at the top, has no parent.
+
+![Tree](https://upload.wikimedia.org/wikipedia/commons/f/f7/Binary_tree.svg)
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Tree_(data_structure))