Add Contribute section to readme.md

Author: wooorm

readme.md

@@ -124,6 +124,14 @@ Parent node of `nodes` ([`Node`][node]).
 
 The matched pattern (`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]
@@ -153,3 +161,7 @@ The matched pattern (`string`).
 [normalize]: https://github.com/syntax-tree/nlcst-normalize
 
 [fn-handler]: #function-handlernodes-index-parent-pattern
+
+[contribute]: https://github.com/syntax-tree/nlcst/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/nlcst/blob/master/code-of-conduct.md