[doc] added links to discord and discourse

Some folks are not aware that we have a Discourse server in addition to the mailing lists and a Discord server in addition to IRC. So I think we should add that.
These were announced on the mailing list a while ago: https://lists.llvm.org/pipermail/llvm-dev/2019-November/136880.html

Differential Revision: https://reviews.llvm.org/D100943

Author: ChristianKuehnel

llvm/docs/GettingInvolved.rst

@@ -98,7 +98,8 @@ Mailing Lists
 -------------
 
 If you can't find what you need in these docs, try consulting the mailing
-lists.
+lists. In addition to the traditional mailing lists there is also a 
+`Discourse server <https://llvm.discourse.group>`_ available. 
 
 `Developer's List (llvm-dev)`__
   This list is for people who want to be included in technical discussions of
@@ -228,6 +229,12 @@ This channel has several bots.
 * clang-bot - A `geordi <http://www.eelis.net/geordi/>`_ instance running
   near-trunk clang instead of gcc.
 
+In addition to the traditional IRC there is a 
+`Discord <https://discord.com/channels/636084430946959380/636725486533345280>`_ 
+chat server available. To sign up, please use this 
+`invitation link <https://discord.com/invite/xS7Z362>`_.
+  
+
 .. _meetups-social-events:
 
 Meetups and social events