documentation: expand Chain Monitor section in Overview

I like the README as an overview :)

I think [this section](https://github.com/arik-so/SwiftLightning/tree/master/Documentation#chain-monitoring) could use a bit more explanation in the vein of the prior two sections.

I'm hesitant to recommend the ChainWatchInterface because I think @jkczyz is trying to refactor it out and make it private (Jeff am I misinterpreting this?).

As a replacement, maybe something like:
"This brings us to chain monitoring. We need a way to tell Rust Lightning about new blocks and their relevant transactions. For this you'll need to instantiate a BlockNotifier struct, which handles informing internal Rust Lightning data structures about newly (dis)connected and relevant transactions within the blocks."

I changed it from saying "the chain monitor" because there's no ChainMonitor per se..

Start with the README Overview and the linked Documentation#chain-monitoring section, then compare it with the prior two sections. Expand the Chain Monitor explanation using the suggested BlockNotifier framing, avoid recommending ChainWatchInterface, and ensure the overview accurately describes informing Rust Lightning about new blocks and relevant transactions.

Written by the indexing model from the issue text.