Add Contribute section to readme.md

Author: wooorm

readme.md

@@ -220,6 +220,14 @@ Yields:
 *   ‡ — Not supported by the underlying algorithm
 *   § — Not very interested in writing / including the code for this
 
+## Contribute
+
+See [`contribute.md` in `syntax-tree/hast`][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]
@@ -245,3 +253,7 @@ Yields:
 [element]: https://github.com/syntax-tree/hast#element
 
 [support]: #support
+
+[contribute]: https://github.com/syntax-tree/hast/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/hast/blob/master/code-of-conduct.md