Contributing.md should direct to the mailing lists

https://github.com/openjdk/jfx/blob/master/CONTRIBUTING.md should mention that the discussions on the issue should happen on the mailing lists. It should link to both the dev and the discuss lists and explain when to use each.
Currently, only the dev list is mentioned and only at the RFR stage.

Some ideas:

1. Under "Feature requests", before "Open an issue on our issues list" it should be discussed on the discuss list if it's "significant enough", or according to whatever guidelines we have.

2. Under "Contributing code and documentation changes", somewhere in or around "Talk about what you would like to do." should link to the dev list probably. And in or around "Feature requests, in particular, should be discussed ahead of time" should probably link to discuss.