Clarify macro_use is for external crates only

Fixes #97

Author: steveklabnik

src/rust-2018/macros/macro-changes.md

@@ -43,6 +43,9 @@ fn main() {
 
 This moves `macro_rules` macros to be a bit closer to other kinds of items.
 
+Note that you'll still need `#[macro_rules]` to use macros you've defined
+in your own crate; this feature only works for importing macros from
+external crates.
 
 ## Procedural macros