Add note regarding perpetual diffs in consul_config_entry documentati…

…on (#288)

Closes #281

Co-authored-by: Blake Covarrubias <blake.covarrubias@gmail.com>

website/docs/r/config_entry.markdown

@@ -12,6 +12,10 @@ The [Configuration Entry](https://www.consul.io/docs/agent/config_entries.html)
 resource can be used to provide cluster-wide defaults for various aspects of
 Consul.
 
+~> **NOTE:** Because the schema in a `consul_config_entry` resource can very
+widely between the various configuration entry kinds, it is necessary to explicitly
+define every attribute to avoid Terraform reporting a diff on the resource.
+
 ## Example Usage
 
 ```hcl