feat(macros-readme): Add link to MacroKata

exercises/macros/README.md

@@ -4,6 +4,10 @@ Rust's macro system is very powerful, but also kind of difficult to wrap your
 head around. We're not going to teach you how to write your own fully-featured
 macros. Instead, we'll show you how to use and create them.
 
+If you'd like to learn more about writing your own macros, the 
+[macrokata](https://github.com/tfpk/macrokata) project has a similar style
+of exercises to Rustlings, but is all about learning to write Macros.
+
 ## Further information
 
 - [Macros](https://doc.rust-lang.org/book/ch19-06-macros.html)