添加详细构建指南 (#4524)

docs/Building.md

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+# Build Guide
+
+<!-- #BEGIN LANGUAGE_SWITCHER -->
+**English** | [中文](Building_zh.md)
+<!-- #END LANGUAGE_SWITCHER -->
+
+## Requirements
+
+To build the HMCL launcher, you need to install JDK 17 (or higher). You can download it here: [Download Liberica JDK](https://bell-sw.com/pages/downloads/#jdk-25-lts).
+
+After installing the JDK, make sure the `JAVA_HOME` environment variable points to the required JDK directory.
+You can check the JDK version that `JAVA_HOME` points to like this:
+
+<details>
+<summary>Windows</summary>
+
+PowerShell:
+```
+PS > & "$env:JAVA_HOME/bin/java.exe" -version
+openjdk version "25" 2025-09-16 LTS
+OpenJDK Runtime Environment (build 25+37-LTS)
+OpenJDK 64-Bit Server VM (build 25+37-LTS, mixed mode, sharing)
+```
+
+</details>
+
+<details>
+<summary>Linux/macOS/FreeBSD</summary>
+
+```
+> $JAVA_HOME/bin/java -version
+openjdk version "25" 2025-09-16 LTS
+OpenJDK Runtime Environment (build 25+37-LTS)
+OpenJDK 64-Bit Server VM (build 25+37-LTS, mixed mode, sharing)
+```
+
+</details>
+
+## Get HMCL Source Code
+
+- You can get the latest source code via [Git](https://git-scm.com/downloads):
+  ```shell
+  git clone https://github.com/HMCL-dev/HMCL.git
+  cd HMCL
+  ```
+- You can manually download a specific version of the source code from the [GitHub Release page](https://github.com/HMCL-dev/HMCL/releases).
+
+## Build HMCL
+
+To build HMCL, switch to the root directory of the HMCL project and run the following command:
+
+```shell
+./gradlew clean makeExecutables
+```
+
+The built HMCL program files are located in the `HMCL/build/libs` subdirectory under the project root.