docs: added info about version/revision to README.md

Author: ReuDa

README.md

@@ -79,3 +79,10 @@ After installing, configure the extension by editing `/etc/steadybit/extension-i
 Make sure that the extension is registered with the agent. In most cases this is done automatically. Please refer to
 the [documentation](https://docs.steadybit.com/install-and-configure/install-agent/extension-registration) for more
 information about extension registration and how to verify.
+
+## Version and Revision
+
+The version and revision of the extension:
+- are printed during the startup of the extension
+- are added as a Docker label to the image
+- are available via the `version.txt`/`revision.txt` files in the root of the image