add CONTRIBUTING file

Author: miniupnp

CONTRIBUTING

@@ -0,0 +1,11 @@
+MiniUPnP Project
+
+Before reporting a bug, please test using the latest development sources
+available on github : https://github.com/miniupnp/miniupnp/
+Have a look at existing bug reports on github 
+https://github.com/miniupnp/miniupnp/issues
+and on the forum https://miniupnp.tuxfamily.org/forum/
+
+Please prefer the github repository to report bugs, send patches, etc.
+
+Thomas Bernard