Merge pull request #450 in ZP/z-push from feature/ZP-1142-readme to develop

* commit 'de25acc6':
  ZP-1142 remove double using. Released under the Affero GNU General Public License (AGPL) version 3.
  ZP-1142 update based on remarks in pull request. Released under the Affero GNU General Public License (AGPL) version 3.
  ZP-1142 add readme and contributing file. Released under the Affero GNU General Public License (AGPL) version 3.

CONTRIBUTING.md

+# How to contribute to Z-Push
+
+Please see the [Z-Push Wiki](https://wiki.z-hub.io/display/ZP/Development+guidelines) for contribution information.
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README.md

+# Z-Push
+Z-Push is an open-source application to synchronize ActiveSync compatible devices such as mobile phones, tablets and Outlook 2013 and above. With a history of almost 10 years of successful synchronization with multiple backends Z-Push is the leading open source push synchronization.
+
+Z-Push can be easily installed using our package repositories, see the [install instructions](https://wiki.z-hub.io/display/ZP/Installation) for more information.
+
+# Contributing
+Please see the [Z-Push Wiki](https://wiki.z-hub.io/display/ZP/Development+guidelines) for contribution information.
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