Add a readme with really basic info.

I do not expect the readme to contain much, it will mostly link to the
documentation, which should become really complete.

README.rst

@@ -0,0 +1,27 @@
+.. Can we write README's in ReST also for use on Gitea? Let's see :-)
+
+Birdvisu
+@@@@@@@@
+
+Birdvisu is a library and a graphical app to visualize routing topologies of
+OSPF. It does not implement OSPF on its own, but rather exports current state
+from `BIRD`_ – that's where the name comes from.
+
+Also, this is the project for my Bachelor thesis.
+
+.. TODO: link the thesis, when it is done.
+
+.. _BIRD: https://bird.network.cz/
+
+Documentation
+=============
+
+Documentation is built using `Sphinx`_ by running ``make`` in the ``docs/`` directory.
+
+.. _Sphinx: https://www.sphinx-doc.org/
+
+Licence
+=======
+
+I have not decided yet, so please ask for terms if you want to use this in some
+non-trivial way. Obviously this is source available :-)