Merge pull request #98 from rust-lang-nursery/guide-extern-c

guide: use "C" in `extern` items

Author: nrc

guide/items.md

@@ -256,3 +256,9 @@ where
     body
 }
 ```
+
+### extern items
+
+When writing extern items (such as `extern "C" fn`), always be explicit about
+the ABI. For example, write `extern "C" fn foo ...`, not `extern fn foo ...`, or
+`extern "C" { ... }.