README for this exercise.
Chris Biscardi: [0:00] 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."
[0:12] 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.