Add Contribute section to readme.md

Author: wooorm

readme.md

@@ -75,6 +75,14 @@ When strings are encountered, they are normalised to [`text`][text] nodes.
 
 [`Element`][element].
 
+## 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]
@@ -104,3 +112,7 @@ When strings are encountered, they are normalised to [`text`][text] nodes.
 [hyperscript]: https://github.com/dominictarr/hyperscript
 
 [text]: https://github.com/syntax-tree/unist#text
+
+[contribute]: https://github.com/syntax-tree/hast/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/hast/blob/master/code-of-conduct.md