Add zig implementation link (#216)

Author: thechampagne

README.md

@@ -93,6 +93,7 @@ Other implementations of Ryu:
 | D                | Ilya Yaroshenko    | [https://github.com/libmir/mir-algorithm][5]  |
 | Scala            | Denys Shabalin     | [https://github.com/scala-native/scala-native][4] |
 | Erlang/BEAM      | Thomas Depierre    | https://github.com/erlang/otp/tree/master/erts/emulator/ryu |
+| Zig              | Marc Tiehuis       | https://github.com/tiehuis/zig-ryu            |
 
 
 [1]: https://dl.acm.org/citation.cfm?id=93559