Add Contribute section to readme.md

Author: wooorm

readme.md

@@ -68,6 +68,14 @@ Merge affix emoticons into the previous sentence.
 
 The node to process ([`NLCSTParagraphNode`][paragraph]).
 
+## 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]
@@ -91,3 +99,7 @@ The node to process ([`NLCSTParagraphNode`][paragraph]).
 [retext-emoji]: https://github.com/wooorm/retext-emoji
 
 [paragraph]: https://github.com/syntax-tree/nlcst#paragraph
+
+[contribute]: https://github.com/syntax-tree/nlcst/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/nlcst/blob/master/code-of-conduct.md