build: update dotnet installation instructions for macOS / Linux

.tool-versions

@@ -0,0 +1 @@
+dotnet 6.0.419

README.md

@@ -1,7 +1,38 @@
+# C# 程式碼範例集合
+
+這是一個簡單的 C# 程式碼範例集合。
+
+### 安裝 . NET SDK
+
+* Windows
+
+```bash
+choco install -y dotnet-6.0-sdk dotnet-8.0-sdk
 ```
-dotnet new console --framework net6.0 --use-program-main -n <project-name>
+
+* macOS / Linux
+
+```bash
+# asdf plugin add dotnet https://github.com/hensou/asdf-dotnet.git
+asdf plugin add dotnet
+asdf install dotnet 6.0.419
+asdf install dotnet 8.0.201
+asdf local dotnet 6.0.419
+dotnet --version
 ```
 
+### 建立專案
+
+```bash
+dotnet new console --framework net6.0 --use-program-main -n <project-name>
 ```
+
+```bash
 dotnet sln add <project-name>/<project-name>.csproj
 ```
+
+### 執行專案
+
+```bash
+dotnet run --project <project-name>/<project-name>.csproj
+```