add doc for LogReader in README.md (#892)

* add doc for LogReader in README.md

* fix review

README.md

@@ -221,6 +221,38 @@ After launching the panel by one of the above methods, developers can see the vi
   <img src="https://user-images.githubusercontent.com/48054808/90868674-ba321f00-e3c9-11ea-83c1-f03c6dd19187.png" width="70%"/>
 </p>
 
+### 3. Read data in log files using LogReader
+
+VisualDL also provides `LogReader` interface to read raw data from log files.
+
+```python
+class LogReader(file_path='')
+```
+#### interface parameters
+| parameters | type   | meaning                              |
+| ---------- | ------ | ------------------------------------ |
+| file_path  | string | File path of the log file. Required. |
+#### Example
+If there is a log file named `vdlrecords.1605533348.log` in the directory of `./log`, we can retrieve the data under the 'loss' tag in the scalar by:
+```python
+from visualdl import LogReader
+reader = LogReader(file_path='./vdlrecords.1605533348.log')
+data = reader.get_data('scalar', 'loss')
+print(data)
+```
+The result will be a list shown as below：
+```python
+...
+id: 5
+tag: "Metrics/Training(Step): loss"
+timestamp: 1605533356039
+value: 3.1297709941864014
+...
+```
+
+For more information of `LogReader`, please refer to [LogReader](./docs/io/LogReader.md).
+
+
 ## Function Preview
 
 ### Scalar

README_CN.md

@@ -230,25 +230,23 @@ app.run(logdir="./log")
 VisualDL的后端也提供了获取日志数据的组件`LogReader`，可通过其获取日志中任意数据，接口如下：
 
 ```python
-class LogReader(logdir=None,
-                file_name='')
+class LogReader(file_path='')
 ```
 
 #### 接口参数
 
-| 参数      | 格式   | 含义                       |
-| --------- | ------ | -------------------------- |
-| logdir    | string | 日志文件所在的路径，必填   |
-| file_name | string | 指定要读的日志文件名，必填 |
+| 参数            | 格式    | 含义                                                         |
+| --------------- | ------- | ------------------------------------------------------------ |
+| file_path       | string  | 指定要读的日志文件路径，必填，注意这里与file_name不同，需填写具体路径|
 
 #### 示例
 
-假定在`./log`文件夹下有一个日志文件`vdlrecords.1605533348.log`，则获取此日志中tag为`loss`的scalar数据过程如下：
+假定在`./log`文件夹下有一个日志文件`vdlrecords.1605533348.log`，则获取LogReader实例如下：
 
 ```python
 from visualdl import LogReader
 
-reader = LogReader(logdir='./log', file_name='vdlrecords.1605533348.log')
+reader = LogReader(file_path='./log/vdlrecords.1605533348.log')
 data = reader.get_data('scalar', 'loss')
 print(data)
 ```