datasources: Add kafka descriptions

Related to fluent#1.

content/datasources/kafka.md

@@ -0,0 +1,31 @@
+# Collecting Data from Kafka
+
+## Scenario
+
+[Kafka](http://kafka.apache.org) is a highly distributed messaging system.
+
+You run Kafka as a messaging system and now want to send the messages into various other systems.
+
+Fluentd can setup to collect messages from Kafka. Applications include:
+
+1. Sending Kafka messages into HDFS for analysis
+2. Sending Kafka messages into Elasticsearch for analysis
+
+## Setup
+
+1. Download the latest [kafka-fluentd-consumer jar](https://github.com/treasure-data/kafka-fluentd-consumer/releases).
+
+2. Set kafka-fluentd-consumer settings correctly. (See [fluentd-consumer.properties](https://github.com/treasure-data/kafka-fluentd-consumer/blob/master/config/fluentd-consumer.properties) for example.)
+
+3. Open your Fluentd configuration file and add the following lines:
+
+    ```
+    <source>
+      type exec
+      command java -Dlog4j.configuration=file:///path/to/log4j.properties -jar /path/to/kafka-fluentd-consumer-LATEST_VERSION-all.jar /path/to/config/fluentd-consumer.properties
+      tag dummy
+      format json
+    </source>
+    ```
+
+   With the above setup, Fluentd consumes Kafka messages which are specified topics in `fluentd-consumer.properties` via `in_exec` plugin.