docs: add build guide

README.md

@@ -1,2 +1,21 @@
 ## What's it
 A cube game like Minecraft, using C++ and OpenGL.  
+
+## Build Guide  
+### Prerequisites  
+- **CMake** (>= 3.24)
+- **C++ 23** compatible compiler
+### Step 
+1. Clone the repository  
+   ```bash
+   git clone https://github.com/zhenyan121/Cubed.git && cd Cubed
+   ```
+2. Configure with CMake
+   ```bash
+   mkdir build && cd build
+   cmake -G "Ninja" ..
+   ```
+3. Build the project  
+   ```bash
+   ninja
+   ```