Update README.md

Author: lemire

README.md

@@ -14,6 +14,9 @@ The following filter types are currently implemented:
 * Xor filter: 8 and 16 bit variants; needs less space than cuckoo filters, with faster lookup
 * Xor+ filter: 8 and 16 bit variants; compressed xor filter
 
+Reference: [Xor Filters: Faster and Smaller Than Bloom and Cuckoo Filters](https://arxiv.org/abs/1912.08258), Journal of Experimental Algorithmics (to appear).
+
+
 ## Password Lookup Tool
 
 Included is a tool to build a filter from a list of known password (hashes), and a tool to do lookups. That way, the password list can be queried locally, without requiring a large file. The filter is only 650 MB, instead of the original file which is 11 GB. At the cost of some false positives (unknown passwords reported as known, with about 1% probability).