Add Contribute section to readme.md

Author: wooorm

readme.md

@@ -92,6 +92,14 @@ around these differences for:
 
 *   [`hastscript`][hastscript]
 
+## Contribute
+
+See [`contribute.md` in `syntax-tree/hast`][contribute] for ways to get
+started.
+
+This organisation has a [Code of Conduct][coc].  By interacting with this
+repository, organisation, or community you agree to abide by its terms.
+
 ## License
 
 [MIT][license] © [Titus Wormer][author]
@@ -125,3 +133,7 @@ around these differences for:
 [react]: https://facebook.github.io/react/docs/glossary.html#react-elements
 
 [hastscript]: https://github.com/syntax-tree/hastscript
+
+[contribute]: https://github.com/syntax-tree/hast/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/hast/blob/master/code-of-conduct.md