Merge pull request #661 from eulerkochy/add_rb_tree_to_readme

Add RBTree to README.md

Author: eulerkochy

README.md

@@ -30,6 +30,7 @@ This package implements a variety of data structures, including
 -   DataStructures.IntSet
 -   SparseIntSet
 -   DiBitVector (in which each element can store two bits)
+-   Red Black Tree
 
 Resources
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