Added Changelog and Contributors.

Author: rion18

CHANGELOG.MD

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+# Change Log
+
+## [1.0.0](https://github.com/devsu/push-sender/releases/tag/1.1.0) (2015-12-28)
+
+First release of the GCM Push Client.
+
++ Initial commit.
++ Added Javadocs.
++ Added Readme and License
++ Added GCM and APNS push services.

CONTRIBUTORS.TXT

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+This file contains a list of people who have made contributions to Push Sender.
+
+Primary authors:
+
+Alvaro López <rion18@hotmail.com>