Add radixtree link to README

Added link to adaptive radix tree implementation.

Author: gammazero

README.md

@@ -693,6 +693,7 @@ additional ordered map implementations.
 - [mspm](https://github.com/BlackRabbitt/mspm) - Multi-String Pattern Matching Algorithm for information retrieval.
 - [parsefields](https://github.com/MonaxGT/parsefields) - Tools for parse JSON-like logs for collecting unique fields and events.
 - [ptrie](https://github.com/viant/ptrie) - An implementation of prefix tree.
+- [radixtree](https://github.com/gammazero/radixtree) - Adaptive radix tree (prefix-tree or compact-trie).
 - [trie](https://github.com/derekparker/trie) - Trie implementation in Go.
 
 ### Trees