Document Common Macros in src/doc/trpl/macros.md

Author: munds

src/doc/trpl/macros.md

@@ -357,6 +357,16 @@ fn main() {
 
 [items]: ../reference.html#items
 
+# Common Macros
+
+The Rust Programming Language has a few macros that you might find useful. Some of them are:
+  + panic!
+  + unreachable!
+  + vec!
+  + unimplemented!
+
+and [more](http://doc.rust-lang.org/std/index.html#macros). 
+
 # Further reading
 
 The [advanced macros chapter][] goes into more detail about macro syntax. It