README for this exercise.
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Chris Biscardi: In macros1, we learn how to define our first macro. In this case, we use the macro_rules macro to define a macro with the name my_macro that returns a function that uses the println macro to print "Check out my macro."
The Rust compiler tells us that we cannot find function my_macro in scope. This is because we're not calling a function here. We're calling a macro. Macros are called with an exclamation point.