Add Contribute section to readme.md

Author: wooorm

readme.md

@@ -213,6 +213,14 @@ should however be entirely stripped from the tree.
 ]
 ```
 
+## Contribute
+
+See [`contribute.md` in `syntax-tree/unist`][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]
@@ -240,3 +248,7 @@ should however be entirely stripped from the tree.
 [github]: https://github.com/jch/html-pipeline/blob/master/lib/html/pipeline/sanitization_filter.rb
 
 [schema-github]: lib/github.json
+
+[contribute]: https://github.com/syntax-tree/unist/blob/master/contributing.md
+
+[coc]: https://github.com/syntax-tree/unist/blob/master/code-of-conduct.md