Update repository README

[gdiazlo]

Describe the solution you'd like

The current README has sections which are better covered in our documentation. We need to remove those sections and add pointers to the official documentation site. For example,

• Requisites
• Installation
• Upgrade

We need to update our wording to reflect our current branding and strategy.

Describe alternatives you've considered

One option could be to maintain these instructions in the README and also in the documentation site, but I believe this potentially leads to confusion. Also, this is a sources' repository, and the documentation should be focused to the developers and contributors. For all other potential users, pointers to the relevant user documentation could serve them better.