updated mailing list to mailing list and Slack channel

MAILINGLIST.rst

@@ -1,11 +1,19 @@
-============
-Mailing list
-============
+==============================
+Mailing list and Slack channel
+==============================
 
 You can join the **lenstronomy** mailing list by signing up on the
 `google groups page <https://groups.google.com/forum/#!forum/lenstronomy>`_.
 
 The email list is meant to provide a communication platform between users and developers. You can ask questions,
 and suggest new features. New releases will be announced via this mailing list.
 
-If you encounter errors or problems with **lenstronomy**, please let us know!
+We also have a `Slack channel <https://lenstronomers.slack.com>`_ for the community.
+Please send us an `email <[email protected]>`_ such that we can add you to the channel.
+
+If you encounter errors or problems with **lenstronomy**, please let us know!
+You can open an `issue  <https://github.com/lenstronomy/lenstronomy/issues>`_,
+make a post on the `Slack channel <https://lenstronomers.slack.com>`_ or
+write an `email <[email protected]>`_ to the lenstronomy developers.
+
+We are also encouraging you to reach out with feature requests, general or specific feedback and questions about use cases.