Update README.md

Author: lemire

README.md

@@ -170,3 +170,4 @@ It took 0.358220 seconds to build an index over 10000000 values.
 * [Python](https://github.com/GreyDireWolf/pyxorfilter)
 * [C99](https://github.com/skeeto/xf8)
 * [Julia](https://github.com/JokingHero/FastFilter.jl)
+* [C#](https://github.com/jonmat/FastIndex)