Update docs marker-cluster.md

Author: Gugustinette

docs/guide/marker-cluster.md

@@ -1,5 +1,10 @@
 # Using Leaflet.markercluster
 
+::: warning
+This is still not proven as stable because of weird building behavior.
+Feel free to report any issue in the Github repository, this could help a lot improving this feature.
+:::
+
 The guide explains how to use the [Leaflet.markercluster](https://github.com/Leaflet/Leaflet.markercluster) plugin.
 A dedicated composable is available to help you use this plugin.