add syn reverse dependencies

good idea @killercup

Author: steveklabnik

_posts/2017-02-02-Rust-1.15.md

@@ -124,10 +124,14 @@ Programming Language"](https://doc.rust-lang.org/book/procedural-macros.html).
 
 While we've said "Serde and Diesel" a number of times here, there's a lot of
 other cool things you can do with custom derive: see
-[`derive-new`](https://crates.io/crates/derive-new) for another example. Custom
-derive was also known as "macros 1.1", as it includes the infrastructure for
-supporting even more compile-time powers of Rust, nicknamed "macros 2.0."
-Expect to hear more about this space in future releases.
+[`derive-new`](https://crates.io/crates/derive-new) for another example. See
+[the `syn` crate's reverse dependencies for more.][syn-deps] (`syn` is
+important for writing custom derives, see the book chapter, linked above, for
+more.) Custom derive was also known as "macros 1.1", as it includes the
+infrastructure for supporting even more compile-time powers of Rust, nicknamed
+"macros 2.0." Expect to hear more about this space in future releases.
+
+[syn-deps]: https://crates.io/crates/syn/reverse_dependencies
 
 #### Other improvements