[add] Dev Container configuration for auto CDE (#2)

Co-authored-by: South Drifter <shiy2008@gmail.com>

Author: Copilot

.devcontainer/Dockerfile

@@ -0,0 +1,4 @@
+FROM mcr.microsoft.com/devcontainers/cpp:debian
+
+USER vscode
+RUN curl -fsSL https://xmake.io/shget.text | bash

.devcontainer/devcontainer.json

@@ -0,0 +1,19 @@
+{
+  "name": "C++",
+  "build": { "dockerfile": "Dockerfile" },
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "tboox.xmake-vscode",
+        "ms-vscode.cpptools-extension-pack",
+        "yzhang.markdown-all-in-one",
+        "redhat.vscode-yaml",
+        "esbenp.prettier-vscode",
+        "eamodio.gitlens",
+        "github.vscode-pull-request-github",
+        "github.vscode-github-actions",
+        "github.copilot"
+      ]
+    }
+  }
+}

ReadMe.md

@@ -13,11 +13,6 @@ The simplest C/C++ project template for beginners.
 
 2.  Click the **[<kbd>Open in GitHub codespaces</kbd>][2] button** on the top of ReadMe file, then an **online VS Code development environment** will be started immediately
 
-3.  Run following command in VS Code terminal to install XMake:
-    ```bash
-    curl -fsSL https://xmake.io/shget.text | bash
-    ```
-
 ## Install minimal All-in-One Development Environment in your computer
 
 ### Windows