Better document that each shortcut needs to a function (or wrapped in one)

Fortunately, the fix is pretty easy. Each shortcut needs to be encapsulated in a function. The doesn't need to be a named, but it helps and I recommend it:

Hey, @gadenbuie 👋
Maybe you can add this into the README file or in the Get Started section? I have also encountered this issue. Unfortunately, it's not really straightforward from the package documentation that it's better to wrap all shortcuts in functions :(

Originally posted by @atsyplenkov in #24 (comment)

The quick start guide includes this instruction, but it's a little too opaque and the following example shows a packaged function rather than a user-created function.

shrtcts/man/fragments/shrtcts-quick-start.Rmd

Lines 13 to 15 in 41051cf

    
           Each shortcut should look something like the example below, 
        
           but you can include _any R code you want_ as the shortcut, 
        
           as long as it's a function.

	Each shortcut should look something like the example below,
	but you can include _any R code you want_ as the shortcut,
	as long as it's a function.