Add Contributing section to readme.md

Author: wooorm

readme.md

@@ -52,6 +52,14 @@ Detect if [`node`][node] is [**generated**][spec].
 *   [`unist-util-stringify-position`](https://github.com/syntax-tree/unist-util-stringify-position)
     — Stringify a node, position, or point
 
+## 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]
@@ -77,3 +85,7 @@ Detect if [`node`][node] is [**generated**][spec].
 [node]: https://github.com/syntax-tree/unist#node
 
 [spec]: https://github.com/syntax-tree/unist#location
+
+[contribute]: https://github.com/syntax-tree/unist/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/unist/blob/master/code-of-conduct.md