Consider making the main README up-to-date with the latest release

At present, [the main README](https://github.com/avast/retdec#retdec) reflects the state of the current `master`. This is good from the development point of view. However, when there is a breaking change since the last release, users might be confused (see e.g. #859). We should consider making the main README up-to-date with the latest release.

Alternatively, we should mention that the current README corresponds to `master` and provide links to READMEs corresponding to each release. E.g.
* [v4.0](https://github.com/avast/retdec/tree/v4.0#retdec)
* [v3.3](https://github.com/avast/retdec/tree/v3.3#retdec)
* ...

However, we will then have to ensure that README for a release does not state that it is for `master` :slightly_smiling_face:

Start with the main README and the v4.0 and v3.3 release README links named in the issue. Decide whether the main README should match the latest release or identify itself as master and link release versions. Done means the chosen approach is applied consistently and release READMEs no longer incorrectly describe themselves as master.

Written by the indexing model from the issue text.