add hint to gcs.md

Author: georgexu99

config/publishers/gcs.md

@@ -1,5 +1,9 @@
 # Google Cloud Storage
 
+{% hint style="warning" %}
+This publish target was added in Electron Forge 7.1.0.
+{% endhint %}
+
 The Google Cloud Storage (GCS) target publishes all your artifacts to a [Google Cloud Storage bucket](https://cloud.google.com/storage/docs), nothing fancy here, literally just puts all your artifacts straight into the bucket.
 
 {% hint style="warning" %}