Update README.md

Added repo instructions

README.md

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 # FCM Client [![CircleCI](https://circleci.com/gh/ToxicBakery/fcm-client/tree/master.svg?style=svg)](https://circleci.com/gh/ToxicBakery/fcm-client/tree/master) [![codecov](https://codecov.io/gh/ToxicBakery/fcm-client/branch/master/graph/badge.svg)](https://codecov.io/gh/ToxicBakery/fcm-client)
-Kotlin library for sending FCM messages.
+Kotlin library for sending Firebase Cloud Messages using the v1 HTTP protocol.
+
+## Requirements
+Sending of messages to FCM requires the admin service account credentials.
+* Navigate to your project in the Firebase console
+* Click the settings gear icon and then `Project settings` in the drop down
+* Click the `Service Accounts` tab
+* Click `Generate new private key` on the `Firebase Admin SDK` tab
+
+## Sample Usage
+```kotlin
+// Create the client using the downloaded google-services.json
+val fcmClient = File("google-services.json")
+   .inputStream()
+   .let { accountStream -> GoogleServiceAccountResolver(accountStream) }
+   .let { accountResolver -> HttpUrlFcmClient(accountResolver) }
+
+// Create a message and give it to the client for delivery to FCM
+fcmClient.send(
+   Message(
+      data = mapOf(
+         "Hello" to "World!"
+      ),
+      android = AndroidConfig(
+         restrictedPackageName = "com.sample.package"
+      ),
+      condition = "'my-channel' in topics"
+   )
+)
+```
+
+## Install
+```groovy
+implementation "com.ToxicBakery.fcmclient:fcm-client:1.+"
+```