[DOCS] Adds feature list to overview.

Author: szabosteve

docs/overview.asciidoc

@@ -10,6 +10,17 @@ Further resources:
 * https://docs.rs/elasticsearch[API documentation]
 
 
+[discrete]
+[[features]]
+=== Features
+
+* Fluent builders for all {es} REST API endpoints
+* Persistent keep-alive connections
+* TLS support with system or custom certificates
+* Proxy support with authentication
+* Async support with Tokio
+
+
 [discrete]
 === {es} Version Compatibility