update README.md

- add links to the community forum
- remove references to the mailing list (as suggested by Riya)

Closes #5263
//FREEBIE

README.md

@@ -29,20 +29,16 @@ https://www.transifex.com/projects/p/signal-android/
 ## Contributing Code
 Instructions on how to setup your development environment and build Signal can be found in  [BUILDING.md](https://github.com/WhisperSystems/Signal-Android/blob/master/BUILDING.md).
 
-If you're new to the Signal codebase, we recommend going through our issues and picking out a simple bug to fix (check the "easy" label in our issues) in order to get yourself familiar.
+If you're new to the Signal codebase, we recommend going through our issues and picking out a simple bug to fix (check the "easy" label in our issues) in order to get yourself familiar. Also please have a look at the [CONTRIBUTING.md](https://github.com/WhisperSystems/Signal-Android/blob/master/CONTRIBUTING.md), that might answer some of your questions.
 
-For larger changes and feature ideas, we ask that you propose it on the mailing list for a high-level discussion before implementation.
+For larger changes and feature ideas, we ask that you propose it on the [unofficial Community Forum](https://whispersystems.discoursehosting.net) for a high-level discussion with the wider community before implementation.
 
 This repository is set up with [BitHub](https://whispersystems.org/blog/bithub/), so you can make money for committing to Signal. The current BitHub price for an accepted pull request is:
 
 [![Current BitHub Price](https://bithub.herokuapp.com/v1/status/payment/commit/)](https://whispersystems.org/blog/bithub/)
 
 ## Contributing Ideas
-Have something you want to say about Open Whisper Systems projects or want to be part of the conversation? Get involved in the mailing list!
-
-whispersystems@lists.riseup.net
-
-https://lists.riseup.net/www/info/whispersystems
+Have something you want to say about Open Whisper Systems projects or want to be part of the conversation? Get involved in the [community forum](https://whispersystems.discoursehosting.net).
 
 ## Contributing Funds
 [![Donate](https://cloud.githubusercontent.com/assets/3121306/11278543/d46e03d0-8eeb-11e5-9691-0da1bf643192.png)](https://www.coinbase.com/checkouts/51dac699e660a4d773216b5ad94d6a0b)