[ISSUE #4661] Add documentation for slack sink connector. (#4662)

* Add documentation for slack sink connector.

eventmesh-connectors/eventmesh-connector-slack/README.md

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+# eventmesh-connector-slack
+
+## SlackSinkConnector：from eventmesh to slack。
+
+1. launch your eventmesh-runtime.
+2. enable sinkConnector and check `sink-config.yml`.
+3. send a message to eventmesh with the topic defined in `pubSubConfig.subject`
+```yaml
+pubSubConfig:
+  meshAddress: 127.0.0.1:10000
+  subject: TEST-TOPIC-SLACK
+  idc: FT
+  env: PRD
+  group: slackSink
+  appId: 5034
+  userName: slackSinkUser
+  passWord: slackPassWord
+sinkConnectorConfig:
+  connectorName: slackSink
+  # Please refer to: https://api.slack.com/messaging/sending
+  appToken: slackAppToken
+  channelId: slackChannelId
+```

eventmesh-connectors/eventmesh-connector-slack/README_CN.md

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+# eventmesh-connector-slack
+
+## SlackSinkConnector：从 eventmesh 到 slack。
+
+1. 启动你的 eventmesh-runtime。
+2. 启用 sinkConnector 并检查 `sink-config.yml`。
+3. 向 eventmesh 发送带有在 `pubSubConfig.subject` 中定义的主题消息。
+```yaml
+pubSubConfig:
+  meshAddress: 127.0.0.1:10000
+  subject: TEST-TOPIC-SLACK
+  idc: FT
+  env: PRD
+  group: slackSink
+  appId: 5034
+  userName: slackSinkUser
+  passWord: slackPassWord
+sinkConnectorConfig:
+  connectorName: slackSink
+  # 以下配置请参考文档：https://api.slack.com/messaging/sending
+  appToken: slackAppToken
+  channelId: slackChannelId
+```