Add Contribute section to readme.md

Author: wooorm

readme.md

@@ -45,6 +45,14 @@ Check if the given value is a [**transparent**][spec] [element][].
 
 `boolean` — whether `node` passes the test.
 
+## 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]
@@ -70,3 +78,7 @@ Check if the given value is a [**transparent**][spec] [element][].
 [element]: https://github.com/syntax-tree/hast#element
 
 [spec]: https://html.spec.whatwg.org/#transparent-content-models
+
+[contribute]: https://github.com/syntax-tree/hast/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/hast/blob/master/code-of-conduct.md