Merge pull request #1172 from tarcieri/adx-target-feature

Document ADX `target_feature`

src/attributes/codegen.md

@@ -82,6 +82,7 @@ functions][unsafe function].
 
 Feature     | Implicitly Enables | Description
 ------------|--------------------|-------------------
+`adx`       |          | [ADX] — Multi-Precision Add-Carry Instruction Extensions
 `aes`       | `sse2`   | [AES] — Advanced Encryption Standard
 `avx`       | `sse4.2` | [AVX] — Advanced Vector Extensions
 `avx2`      | `avx`    | [AVX2] — Advanced Vector Extensions 2
@@ -108,6 +109,7 @@ Feature     | Implicitly Enables | Description
 
 <!-- Keep links near each table to make it easier to move and update. -->
 
+[ADX]: https://en.wikipedia.org/wiki/Intel_ADX
 [AES]: https://en.wikipedia.org/wiki/AES_instruction_set
 [AVX]: https://en.wikipedia.org/wiki/Advanced_Vector_Extensions
 [AVX2]: https://en.wikipedia.org/wiki/Advanced_Vector_Extensions#AVX2