update README

[matteodelucchi]

Some parts of the README are outdated, though not incorrect and therefore not (yet) a critical issue. This makes it a good starting point for people who want to learn how to contribute to the abn R package.

• Address the contradiction between the required R version in DESCRIPTION and README. Consider removing that section from the README, as it requires manual updates.
• Update the installation instructions from Github to the most recent version of abn. For example, here.
• Standardize the style of notes, replacing _**Note:**, [!Note], etc., with a consistent style, such as ⚠️.
• Correct typos, grammar, and other errors.
• Expand this incomplete list.