Add missing link to proc-macro2 crate
diff --git a/README.md b/README.md
index 3e48286..bd28f61 100644
--- a/README.md
+++ b/README.md
@@ -241,6 +241,8 @@
 The proc-macro2 crate will automatically detect and use the compiler's data
 structures on sufficiently new compilers.
 
+[proc-macro2]: https://github.com/alexcrichton/proc-macro2
+
 ## License
 
 Licensed under either of