Add kafka plugin docs

docs/Supported-list.md

@@ -30,6 +30,7 @@
   * [gRPC](https://github.com/grpc/grpc-java) 1.6+
 * MQ
   * [RocketMQ](https://github.com/apache/rocketmq) 4.x
+  * [Kafka](http://kafka.apache.org) 0.11.0.0 -> 1.0 (Optional²)
 * NoSQL
   * Redis
     * [Jedis](https://github.com/xetorthio/jedis) 2.x
@@ -47,4 +48,5 @@
   * Motan
   * Hprose-java
 
-¹Required dependencies for these components must be first manually downloaded before being built, due to license incompatibilities. For this reason these components are not by default included in the SkyWalking releases.
+¹Required dependencies for these components must be first manually downloaded before being built, due to license incompatibilities. For this reason these components are not by default included in the SkyWalking releases.
+²These plugin would probably affect the performance because of enhancing class. Or some of them are in beta, If the above plugins are put under the plugins folder, it will cause the user experience not so good, For this reason these plugins is not load by default. 

docs/cn/FAQ/Kafka-plugin-CN.md

@@ -0,0 +1,7 @@
+**现象** 
+Kafka插件无法追踪应用是怎么处理消息
+
+**原因**：Kafka消费端主动从Kafka Cluster获取消息，并且探针无法得知
+消费线程是否被监控.
+
+**解决方法**: 需要手动探针才能解决，获取消息和处理消息的方法追加@Trace，详情见[文档](https://github.com/apache/incubator-skywalking/blob/master/docs/en/Application-toolkit-trace.md)

docs/en/FAQ/kafka-plugin.md

@@ -0,0 +1,8 @@
+**Problem**: <br/>
+The Kafka plug-in can't track how applications handle messages
+
+**Reason**:
+The Kafka consumer actively takes a message from Kafka Cluster and the agent can not see if the consuming thread is being monitored.
+
+**Resolve**:
+Add the `@Trace` annotation to the method of handle message and the method of poll message , see the [document](https://github.com/apache/incubator-skywalking/blob/master/docs/en/Application-toolkit-trace.md)