Added --depth=1 to submodule update to reduce size

BUILDING.md

@@ -5,7 +5,7 @@
 ```bash
 $ git clone https://github.com/paullouisageneau/libdatachannel.git
 $ cd libdatachannel
-$ git submodule update --init --recursive
+$ git submodule update --init --recursive --depth=1
 ```
 
 ## Build with CMake