Add a link for the reference compiletest header (rust-lang#2096)

Author: ehuss

src/doc/rustc-dev-guide/src/tests/directives.md

@@ -249,6 +249,9 @@ See [Pretty-printer](compiletest.md#pretty-printer-tests).
       output pattern
 - [`should-ice`](compiletest.md#incremental-tests) — incremental cfail should
       ICE
+- [`reference`] — an annotation linking to a rule in the reference
+
+[`reference`]: https://github.com/rust-lang/reference/blob/master/docs/authoring.md#test-rule-annotations
 
 ### Tool-specific directives