Add README.

Author: trekhleb

src/data-structures/trie/README.md

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+# Trie
+
+In computer science, a trie, also called digital tree and sometimes 
+radix tree or prefix tree (as they can be searched by prefixes), 
+is a kind of search tree—an ordered tree data structure that is 
+used to store a dynamic set or associative array where the keys 
+are usually strings. Unlike a binary search tree, no node in the 
+tree stores the key associated with that node; instead, its 
+position in the tree defines the key with which it is associated.
+All the descendants of a node have a common prefix of the string
+associated with that node, and the root is associated with the 
+empty string. Values are not necessarily associated with every 
+node. Rather, values tend only to be associated with leaves, 
+and with some inner nodes that correspond to keys of interest. 
+For the space-optimized presentation of prefix tree, see compact 
+prefix tree.
+
+![Trie](https://upload.wikimedia.org/wikipedia/commons/b/be/Trie_example.svg)
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Trie)