Update README

Changes the format of the readme file from Markdown to ReStructuredText
and add links to forum and landing page.

README.rst

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+CADET
+======
+
+.. image:: https://img.shields.io/github/release/modsim/cadet.svg
+   :target: https://github.com/modsim/CADET/releases
+
+.. image:: https://img.shields.io/github/downloads/modsim/cadet/latest/total.svg
+   :target: https://github.com/modsim/CADET/releases
+
+.. image:: https://img.shields.io/travis/modsim/CADET/master.svg?logo=travis&maxAge=3600)
+   :target: https://travis-ci.org/modsim/CADET
+
+.. image:: https://img.shields.io/appveyor/ci/sleweke/cadet/master.svg?logo=appveyor&maxAge=3600svg=true
+   :target: https://ci.appveyor.com/project/sleweke/cadet
+
+- **Website (including documentation):** https://cadet.github.io
+- **Forum:** https://forum.cadet-web.de
+- **Source:** https://github.com/modsim/cadet
+- **Bug reports:** https://github.com/modsim/cadet/issues
+
+
+Download and Installation
+-------------------------
+
+Download the `latest release <https://github.com/modsim/cadet/releases>`_ for your platform.
+`Additional information <https://cadet.github.io/getting_started/installation>`_ and a `tutorial <https://cadet.github.io/getting_started/tutorials/breakthrough>`_ are available to guide you through the installation and the first steps of using CADET.
+
+
+Online Simulator
+----------------
+
+Try CADET in the browser using the `online simulator web service <http://www.cadet-web.de>`_.
+
+
+Ongoing Development
+-------------------
+
+Since CADET is actively developed, do not expect a stable API. Breaking changes and extensive restructuring may occur in any commit and release. For non-developers, it is recommended to upgrade from release to release instead of always working with the most recent commit.
+
+Bugs
+----
+
+Please report any bugs that you find `here <https://github.com/modsim/cadet/issues>`_. Or, even better, fork the repository on `GitHub <https://github.com/modsim/cadet>`_ and create a pull request (PR) with the fix. 
+
+Donations
+---------
+
+`Donations <https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=FCQ2M89558ZAG>`_ for helping to host, maintain, and further develop the CADET project are highly appreciated.
+
+
+License
+----------
+
+Released under GPL v3. License (see `LICENSE.txt <https://github.com/modsim/CADET/blob/master/LICENSE.txt>`_)::
+
+   Copyright (C) 2004-2020 CADET Authors