Add Contributing section to readme.md

Author: wooorm

readme.md

@@ -90,6 +90,14 @@ and the last `end`, and the content inside.
 `Array.<Node>?` — List of nodes to replace `start`, `nodes`, and `end`
 with, optional.
 
+## 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]
@@ -119,3 +127,7 @@ with, optional.
 [remark]: https://github.com/wooorm/remark
 
 [handler]: #function-handlerstart-nodes-end
+
+[contribute]: https://github.com/syntax-tree/mdast/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/mdast/blob/master/code-of-conduct.md