CONTRIBUTING: Add a short file referencing the online doc/contributing page

author: Kim Alvefur <zash@zash.se>
date: Wed, 03 Apr 2019 01:09:21 +0200
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/CONTRIBUTING	Wed Apr 03 01:09:21 2019 +0200
@@ -0,0 +1,9 @@
+Thanks for your interest in contributing to the project!
+
+There are many ways to contribute, such as helping improve the
+documentation, reporting bugs, spreading the word or testing the latest
+development version.
+
+You can find more information on how to contribute at <https://prosody.im/doc/contributing>
+
+See also the HACKERS and README files.
author	Kim Alvefur <zash@zash.se>
date	Wed, 03 Apr 2019 01:09:21 +0200