[DOC] Add QBE dependency installation instructions to README

[Prestgg1]

Issue

The README documents building AzLang but doesn't mention the required system dependencies:

cargo build --release

However, building requires:

• QBE (afterburner backend)
• Linux: binutils (assembler) and ld.lld (linker)
• Windows: MASM/NASM assembler and lld.link

Current State

• Dependencies mentioned in README: Partially
• Installation instructions: Missing

Recommendation

Add a "Building from Source" section with platform-specific instructions:

## Building from Source

### Prerequisites
- Rust toolchain (see rustup.rs)
- QBE: https://c9x.me/compile/
- Linux: `sudo apt install binutils clang lld`
- macOS: `brew install qbe binutils`
- Windows: Install MASM/NASM and LLVM's lld

### Build
```bash
cargo build --release

**Labels:** documentation, good first issue