update README: add installing instruction for .net framework (#65)

lgtm

Author: Aiemu

README.md

@@ -41,6 +41,8 @@ We have prepared Nuget Package for C# users. Users can directly install the clie
 dotnet add package Apache.IoTDB
 ```
 
+Note that the `Apache.IoTDB` package only supports `.net 5.0`. If you are using `.net framework 4.x`, please refer to the section [starting from .net framework 4.x](#starting-from-net-framework-4x).
+
 ## Prerequisites
 
     .NET SDK Version == 5.0 
@@ -65,4 +67,13 @@ NLog >= 4.7.9
 ### Command Line Tools
 
 ## Publish your own client on nuget.org
-You can find out how to publish from this [doc](./PUBLISH.md).
+You can find out how to publish from this [doc](./PUBLISH.md).
+
+## Starting from `.net framework 4.x`
+In order to adapt to `.net framework 4.x`, we have packaged a nuget package separately, the package name is [`Apache.IoTDB.framework`](https://www.nuget.org/packages/Apache.IoTDB.framework/). 
+
+You can install it through Package Manager (PM), .NET CLI, etc. For example (.NET CLI):
+
+```sh
+dotnet add package Apache.IoTDB.framework --version 0.12.1.2
+```

README_ZH.md

@@ -39,6 +39,7 @@ Apache IoTDB Github: https://github.com/apache/iotdb
 dotnet add package Apache.IoTDB
 ```
 
+请注意，`Apache.IoTDB`这个包仅支持`.net 5.0`。 如果您使用的是`.net framework 4.x`，请参考[从`.net framework 4.x`开始](#从-net-framework-4x-开始)。
 
 ## 环境准备
 
@@ -62,4 +63,13 @@ dotnet add package Apache.IoTDB
 ### 命令行工具
 
 ## 在 nuget.org 上发布你自己的客户端
-你可以在这个[文档](./PUBLISH.md)中找到如何发布
+你可以在这个[文档](./PUBLISH.md)中找到如何发布
+
+## 从`.net framework 4.x`开始
+为了适配`.net framework 4.x`，我们单独构建了一个Nuget包，包名是[`Apache.IoTDB.framework`](https://www.nuget.org/packages/Apache.IoTDB.framework/)。
+
+您可以使用PM、.NET CLI等工作来安装它。以.NET CLI为例：
+
+```sh
+dotnet add package Apache.IoTDB.framework --version 0.12.1.2
+```