Adding formal reference.

Author: lemire

README.md

@@ -181,7 +181,7 @@ AMD Rome, Linux, Rust 1.49.
 
 #### References
 
-- Daniel Lemire, [Number Parsing at a Gigabyte per Second](https://arxiv.org/abs/2101.11408), arXiv:2101.11408
+- Daniel Lemire, [Number Parsing at a Gigabyte per Second](https://arxiv.org/abs/2101.11408), Software: Practice and Experience 51 (8), 2021.
 
 #### License