Credit packagecloud

Because we asked packagecloud for more bandwidth, they asked us to
credit them in the README

Author: untitaker

AUTHORS.rst

@@ -14,5 +14,9 @@ In alphabetical order:
 - Michael Adler
 - Thomas Weißschuh
 
-Additionally `FastMail sponsored a paid account for testing
-<https://github.com/pimutils/vdirsyncer/issues/571>`_. Thanks!
+Special thanks goes to:
+
+* `FastMail <https://github.com/pimutils/vdirsyncer/issues/571>`_ sponsors a
+  paid account for testing their servers.
+* `Packagecloud <https://packagecloud.io/>`_ provide repositories for
+  vdirsyncer's Debian packages.

README.rst

@@ -18,6 +18,10 @@ vdirsyncer
   :target: https://pypi.python.org/pypi/vdirsyncer
   :alt: version on pypi
 
+.. image:: https://img.shields.io/badge/deb-packagecloud.io-844fec.svg
+  :target: https://packagecloud.io/pimutils/vdirsyncer
+  :alt: Debian packages
+
 .. image:: https://img.shields.io/pypi/l/vdirsyncer.svg
   :target: https://github.com/pimutils/vdirsyncer/blob/master/LICENCE
   :alt: licence: BSD