[doc] added links to discord and discourse

[kuhnel]

Summary

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

[kuhnel]

rebased on main

[xgupta]

Some folks are not aware that we have a Discourse server in addition to the mailing lists

[As I have seen, experience members of community are not addressing questions on LLVM discourse. It create confusion and newcomers think there are none to answer them.

IMO we should wait for RFC on discourse transition.]

[kuhnel]

We do have those two things in place right now, so IMHO it makes sense to document that.
And users are confused already where to ask their question.
So we should at least be honest and document the current status.

[xgupta]

Agree 😀

llvm/docs/GettingInvolved.rst

:doc:`Release notes for the current release <ReleaseNotes>`
    This describes new features, known bugs, and other limitations.
 
 .. _mailing-lists:
 
 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
   LLVM. People post to this list when they have questions about writing code
   for or using the LLVM tools. It is relatively low volume.
 
   .. __: http://lists.llvm.org/mailman/listinfo/llvm-dev
 
* llvmbb - Bot for the main LLVM buildbot master.
     http://lab.llvm.org:8011/console
   * smooshlab - Apple's internal buildbot master.
 
 * robot - Bugzilla linker. %bug <number>
 
 * 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
 -------------------------
 
 .. toctree::
    :hidden: