Add Contribute section to readme.md

Author: wooorm

readme.md

@@ -97,6 +97,14 @@ Take a look at [`lib/handlers/`][handlers] for examples.
 *   [`rehype-remark`](https://github.com/wooorm/remark-remark)
     — remark support for rehype
 
+## 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]
@@ -130,3 +138,7 @@ Take a look at [`lib/handlers/`][handlers] for examples.
 [handlers]: lib/handlers
 
 [remark-rehype]: https://github.com/wooorm/remark-rehype
+
+[contribute]: https://github.com/syntax-tree/mdast/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/mdast/blob/master/code-of-conduct.md