manifests/README.md: Add initial file version

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

manifests/README.md

+# SPDX-FileCopyrightText: Huawei Inc.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+# `repo` manifests
+
+Setting up a `oniro` workspace is done using the `repo` tool which in turn uses
+the included manifest files for project definitions. These define and set
+upstream components (for example
+[oe-core](https://git.openembedded.org/openembedded-core/)). For detailed
+information on how to setup a `repo` workspace, checkout the
+[documentation](https://docs.oniroproject.org/).