Use original README.md for fluentd plugin explanations

`doc/client/fluentd.m` includes obsolete explanations and it confuses new users.
For example, label_keys option was removed at grafana#1186
but `fluentd.md` has the explanation.

It's simple to maitain `fluentd/fluent-plugin-grafana-loki/README.md` for
the explantion of the output plugin and use the link when we share how to use it.

docs/clients/README.md

@@ -5,7 +5,7 @@ Loki supports the following official clients for sending logs:
 1. [Promtail](./promtail/README.md)
 2. [Docker Driver](./docker-driver/README.md)
 4. [Fluent Bit](../../cmd/fluent-bit/README.md)
-3. [Fluentd](./fluentd.md)
+3. [Fluentd](../../fluentd/fluent-plugin-grafana-loki/README.md)
 
 ## Picking a Client