Added Reactathon 2019 to conferences.md (reactjs#1695)

Author: juanjalvarez

content/community/conferences.md

@@ -16,6 +16,11 @@ Do you know of a local React.js conference? Add it here! (Please keep the list c
 January 31, 2019 in Tehran, Iran
 [Website](http://reactiran.com) - [Instagram](https://www.instagram.com/reactiran/)
 
+### Reactathon 2019 {#reactathon-2019}
+March 30-31, 2019 in San Francisco, USA
+
+[Website](https://www.reactathon.com/) - [Twitter](https://twitter.com/reactathon)
+
 ### App.js Conf 2019 {#appjs-conf-2019}
 April 4-5, 2019 in Kraków, Poland