Checkout submodules with shallow depth (#1353)

* Make submodules shallow

* Updated README for the shallow checkout changes.

README.md

@@ -68,17 +68,16 @@ By default, RCCL builds for all GPU targets defined in `DEFAULT_GPUS` in `CMakeL
 ### To build the library using CMake:
 
 ```shell
-$ git clone https://github.com/ROCm/rccl.git --recurse-submodules
+$ git clone --recursive https://github.com/ROCm/rccl.git
 $ cd rccl
 $ mkdir build
 $ cd build
 $ cmake ..
 $ make -j 16      # Or some other suitable number of parallel jobs
 ```
-If you have already cloned, you can checkout the `mscclpp` submodule manually.
+If you have already cloned, you can checkout the external submodules manually.
 ```shell
-$ cd ext-src/mscclpp
-$ git submodule update --init --recursive
+$ git submodule update --init --recursive --depth=1
 ```
 You may substitute an installation path of your own choosing by passing `CMAKE_INSTALL_PREFIX`. For example:
 ```shell