README.rst: introduce reference to release version 0.1.0-rc1 for docs

Adding a definition we can use throughout the documents and having it
stated in the README.rst as a start.

Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>

README.rst

@@ -49,6 +49,4 @@ A great Open source project wouldn't exist without the hard work of its many con
 
 Release
 *******
-In order to provide new features and bug fixes, the community constantly updates the code and provides new releases.
-
-
+In order to provide new features and bug fixes, the community constantly updates the code and provides new releases. The documentation you are reading matches the |main_project_version| release of OpenHarmony.

definitions.rst

 .. |main_project_name| replace:: OpenHarmony
+.. |main_project_version| replace:: 0.1.0-rc1
 .. |contact_info| replace:: contact@openharmony.io