Add Contributing section to readme.md

Author: wooorm

readme.md

@@ -41,6 +41,14 @@ Get the heading style of a node.
 `string` (`'atx'`, `'atx-closed'`, or `'setext'`) — When an ambiguous
 heading is found, either `relative` or `null` is returned.
 
+## Contribute
+
+See [`contribute.md` in `syntax-tree/mdast`][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]
@@ -68,3 +76,7 @@ heading is found, either `relative` or `null` is returned.
 [mdast]: https://github.com/syntax-tree/mdast
 
 [node]: https://github.com/syntax-tree/unist#node
+
+[contribute]: https://github.com/syntax-tree/mdast/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/mdast/blob/master/code-of-conduct.md