Add Contribute section to readme.md

readme.md

@@ -67,6 +67,13 @@ Check phrases for simpler alternatives.
 *   [`retext-profanities`](https://github.com/retextjs/retext-profanities)
     — Check profane and vulgar wording
 
+## Contribute
+
+See [`contribute.md` in `retextjs/retext`][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]
@@ -88,3 +95,7 @@ Check phrases for simpler alternatives.
 [author]: http://wooorm.com
 
 [retext]: https://github.com/retextjs/retext
+
+[contribute]: https://github.com/retextjs/retext/blob/master/contributing.md
+
+[coc]: https://github.com/retextjs/retext/blob/master/code-of-conduct.md