Add Contributing section to readme.md

Author: wooorm

readme.md

@@ -33,6 +33,14 @@ console.log(size(tree.children[0].children[0])); // 0
 Calculate the number of nodes in [`node`][node].
 The result is excluding the given node itself.
 
+## Contribute
+
+See [`contribute.md` in `syntax-tree/unist`][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]
@@ -56,3 +64,7 @@ The result is excluding the given node itself.
 [unist]: https://github.com/syntax-tree/unist
 
 [node]: https://github.com/syntax-tree/unist#node
+
+[contribute]: https://github.com/syntax-tree/unist/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/unist/blob/master/code-of-conduct.md