administration: configuration: Document configuration modes (#745)

* administration: configuration: list supported configuration formats

Signed-off-by: lecaros <[email protected]>

* administration: configuration: add yaml configuration tech preview message

Signed-off-by: lecaros <[email protected]>

Author: lecaros

administration/configuring-fluent-bit/README.md

@@ -1,2 +1,6 @@
 # Configuring Fluent Bit
 
+Currently, Fluent Bit supports two configuration formats:
+
+* [Classic mode](classic-mode/README.md).
+* [Yaml](yaml/README.md). (YAML configuration is tech preview so not recommended for production.)