Update installation instructions

[wheleph]

Motivation

Our current installation instructions (https://github.com/wheleph/nttt/wiki/Installation-and-usage) start with git clone ... which doesn't guarantee that our customers get a stable tagged version (because that way they just get whatever there is in the branch improvements). It would be much better if users obtained the releases from, well, the Releases page: https://github.com/wheleph/nttt/releases.

Acceptance criteria

• The installation instructions (https://github.com/wheleph/nttt/wiki/Installation-and-usage) are updated in a way that users are guaranteed to use a stable version of NTTT