rough draft of macros chapter

Fixes #1283

Author: steveklabnik

2018-edition/src/SUMMARY.md

@@ -116,6 +116,7 @@
     - [Advanced Traits](ch19-03-advanced-traits.md)
     - [Advanced Types](ch19-04-advanced-types.md)
     - [Advanced Functions & Closures](ch19-05-advanced-functions-and-closures.md)
+    - [Macros](ch19-06-macros.md)
 
 - [Final Project: Building a Multithreaded Web Server](ch20-00-final-project-a-web-server.md)
     - [A Single Threaded Web Server](ch20-01-single-threaded.md)