Add Contribute section to readme.md

Author: wooorm

Diff for: readme.md

@@ -41,6 +41,14 @@ Stringify the given [NLCST][] node (or list of nodes).
 
 `string`.
 
+## Contribute
+
+See [`contribute.md` in `syntax-tree/nlcst`][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]
@@ -62,3 +70,7 @@ Stringify the given [NLCST][] node (or list of nodes).
 [author]: http://wooorm.com
 
 [nlcst]: https://github.com/syntax-tree/nlcst
+
+[contribute]: https://github.com/syntax-tree/nlcst/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/nlcst/blob/master/code-of-conduct.md