add link to GitHub releases to all README files

- the readme files originally mentioned CHANGELOG, but such a file exists only on a tagged version so it might be hard to find when visiting https://github.com/shopsys/shopsys
- updated docs/index.md in the same fashion

Author: vitek-rostislav

README.md

@@ -8,7 +8,7 @@ Shopsys/framework package is part of Shopsys Platform environment and it stands
 
 For more information about project, please see [shopsys/shopsys] readme.
 
-This repository is maintained by [shopsys/shopsys] monorepo, information about changes is in its `CHANGELOG` file.
+This repository is maintained by [shopsys/shopsys] monorepo. See the [releases](https://github.com/shopsys/shopsys/releases) page for the history of changes.
 
 ## How to Start New Project