Add link to library for macos in README

Author: Hiro

ReadMe.md

@@ -54,6 +54,7 @@ To register the GDExtension library with Godot, you need to create two files rel
     [libraries]
     linux.64 = "res://../rust/target/debug/lib{my_ext}.so"
     windows.64 = "res://../rust/target/debug/{my_ext}.dll"
+    macos.64 = "res://../rust/target/debug/{my_ext}.dylib"
     ```
 
 2. A second file `res://.godot/extension_list.cfg` simply lists the path to your first file:
@@ -95,4 +96,4 @@ to talk to the developers in the `#gdext-dev` channel on [Discord]!
 [Godot]: https://godotengine.org
 [`gdnative`]: https://github.com/godot-rust/godot-rust
 [mpl]: https://www.mozilla.org/en-US/MPL/
-[Discord]: https://discord.gg/aKUCJ8rJsc
+[Discord]: https://discord.gg/aKUCJ8rJsc