Add readme

Author: tulir

README.md

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+# gomuks/push
+This is a simple push gateway to let the [gomuks backend](https://github.com/tulir/gomuks)
+send FCM push notifications to [gomuks android](https://github.com/gomuks/android).
+
+The default instance is at push.gomuks.app, but it's probably possible to self-host by forking
+the android app and using your own FCM credentials.