Add root CMakeLists.txt to better support FetchContent (#64)

CMake's `FetchContent_MakeAvailable()` treats the fetched repository as
a CMake project and automatically calls `subdirectory()` on it, if a
`CMakeLists.txt` exists in the root directory.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>

CMakeLists.txt

@@ -0,0 +1,9 @@
+# Copyright (c) 2022, Arm Limited and Contributors. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+# This CMakeLists.txt in the root of the repository ensures that, when another
+# project fetches CMSIS-DSP by calling FetchContent_MakeAvailable(), the call
+# automatically adds CMSIS-DSP as a subdirectory. To maintain compatibility with
+# existing projects that assume CMSIS-DSP's entry point to be Source/, we only
+# add a redirection in the root CMakeLists.txt.
+add_subdirectory(Source)