Update installation page to become a Quick Start guide

In README: Link to Quick Start install page, position BPMN.iO as recommended modeller (on drupal.org) the following tasks were agreed on, where the two first tasks were completed on drupal.org:

1. Single out BPMN as the recommended modeller in the README file
2. Add link to a download and install Quick Start at https://ecaguide.org/eca/install/ in the README file
3. Update https://ecaguide.org/eca/install/ page to become a Quick Start guide

I have removed this redundant section, since ECA is stable. Also, this is basic Composer konwledge:

Using non-stable releases

If you want to use non-stable releases (e.g. RC, beta or dev), then you should configure your root project to allow that by configuring the minimum acceptable stability with e.g. composer config minimum-stability beta for accepting beta releases. Without this, composer may run into dependency issues.