Remove duplicated installation instructions from README

[erak]

Fixes #254.

In conjuction with patch hnrkmrkhwif in radicle.xyz

Motivation

Since the Homebrew / Ubuntu installation instructions in README.md are different from the ones on https://radicle.xyz/get-started.html, I propose to remove the "recommended" instructions documented in README.md and link to the getting started guide instead.

Even if corrected now, differences may occur often in the future, which is why this PR could help to remove the need to update more than one source.

In this repository we'd only keep the instructions for contributors via source / docker.