doc: Stop advising the creation of Phabricator issues

We have noticed that Phabricator (the ticketing system on tf.org [1])
has far less visibility within the community than the mailing list [2].
For this reason, let's drop usage of Phabricator for anything else than
bug reports. For the rest, advise contributors to start a discussion on
the mailing list, where they are more likely to get feedback.

[1] https://developer.trustedfirmware.org/project/board/1/
[2] https://lists.trustedfirmware.org/mailman/listinfo/tf-a

Change-Id: I7d2d3d305ad0a0f8aacc2a2f25eb5ff429853a3f
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/docs/process/contributing.rst b/docs/process/contributing.rst
index 7886cf4..5e490a9 100644
--- a/docs/process/contributing.rst
+++ b/docs/process/contributing.rst
@@ -6,12 +6,16 @@
 
 -  Make sure you have a Github account and you are logged on both
    `developer.trustedfirmware.org`_ and `review.trustedfirmware.org`_.
--  Create an `issue`_ for your work if one does not already exist. This gives
-   everyone visibility of whether others are working on something similar.
 
-   -  If you intend to include Third Party IP in your contribution, please
-      raise a separate `issue`_ for this and ensure that the changes that
-      include Third Party IP are made on a separate topic branch.
+-  If you plan to contribute a major piece of work, it is usually a good idea to
+   start a discussion around it on the mailing list. This gives everyone
+   visibility of what is coming up, you might learn that somebody else is
+   already working on something similar or the community might be able to
+   provide some early input to help shaping the design of the feature.
+
+   If you intend to include Third Party IP in your contribution, please mention
+   it explicitly in the email thread and ensure that the changes that include
+   Third Party IP are made in a separate patch (or patch series).
 
 -  Clone `Trusted Firmware-A`_ on your own machine as described in
    :ref:`prerequisites_get_source`.
@@ -29,8 +33,7 @@
       Makefile target is provided for convenience.
 
 -  Keep the commits on topic. If you need to fix another bug or make another
-   enhancement, please create a separate `issue`_ and address it on a separate
-   topic branch.
+   enhancement, please address it on a separate topic branch.
 -  Avoid long commit series. If you do have a long series, consider whether
    some commits should be squashed together or addressed in a separate topic.
 -  Make sure your commit messages are in the proper format. If a commit fixes