add mobile center build badge

Author: Trolldemorted

README.md

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+[![Build Status](https://build.mobile.azure.com/v0.1/apps/cbdae38f-5ebb-4b11-ac7d-3d4b2648c50b/branches/master/badge)](https://build.mobile.azure.com/v0.1/apps/cbdae38f-5ebb-4b11-ac7d-3d4b2648c50b/branches/master/badge)
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 # Signal-Windows
 
 This is an unofficial Signal client for the Universal Windows Platform. It is (currently) a hobby project in a very early stage of production, so expect bugs and non-backwards compatible changes.