Add app.js Conf to conferences (#2458)

* Add app.js conf

* Change spelling of Kraków

Author: tchayen

content/community/conferences.md

@@ -12,6 +12,11 @@ Do you know of a local React.js conference? Add it here! (Please keep the list c
 
 ## Upcoming Conferences {#upcoming-conferences}
 
+### App.js Conf 2020 {#appjsonf2020}
+April 23 - 24, 2020 in Kraków, Poland
+
+[Website](http://appjs.co/react) - [Twitter](https://twitter.com/appjsconf)
+
 ### React Summit 2019 {#reactsummit2019}
 November 30, 2019 in Lagos, Nigeria