Document features that were considered but deemed out of scope

[JayPanoz]

Given discussions can sometimes be extensive (cf. #83) and spread across several channels (issues, PRs, Slack, engineering calls), it seems to me it could be useful to add a new chapter in docs for features that were considered at some point in time but were then deemed out of scope.

At first sight, that could help list/reference these features, and explain why they did not make it into ReadiumCSS – it might also perhaps help inform decisions on CSS injection in navigators + user settings API.