added env file; updated README

.env

@@ -0,0 +1,2 @@
+# follow the instructions in the [Firebase Documentation](https://firebase.google.com/docs/cloud-messaging/auth-server#provide-credentials-manually) to create a service account. after you create a service account, and download the json file. then change the value of `GOOGLE_APPLICATION_CREDENTIALS` to the path of the json file you downloaded.
+GOOGLE_APPLICATION_CREDENTIALS="/path/to/your/firebase/file.json"

README.md

@@ -8,10 +8,15 @@
 [travis]: https://travis-ci.org/panicbit/fcm-rust
 [coveralls]: https://coveralls.io/github/panicbit/fcm-rust
 
-## Examples
+## v1 API
 
 This fork is a rewrite to use Google's HTTP v1 API.
 
+# Credentials
+
+This library expects the Google credentials JSON location to be 
+defined as `GOOGLE_APPLICATION_CREDENTIALS` in the `.env` file.
+
 ## Examples
 
 Check out the examples directory for a simple sender.