BUILD_UNIX.md: add commands to checkout the submodule(s)
@@ -46,6 +46,7 @@ To build the programs from the source code, run the following commands:
```bash
git clone https://github.com/SoftEtherVPN/SoftEtherVPN.git
+git submodule init && git submodule update
cd SoftEtherVPN
./configure
make -C tmp
