Document implementation support for selected feature flags

[ysangkok]

Feedback appreciated.

[t-bast]

Thanks, I think this is useful! I think each section should contain:

1. A high-level, noob-friendly description of what the feature achieves (not done for all features yet)
2. Historical details about activation in main implementations (which you already did)

I'd also like to see the following features mentioned:

• upfront_shutdown_script: even though eclair still doesn't support it and it's not commonly used, this can be a useful feature for some users
• gossip_queries / gossip_queries_ex (I think we can have a single paragraph for both, as both should generally be enabled together)
• It's probably a bit too early to mention anchor outputs

[ysangkok]

@t-bast I pushed a commit that addresses all your comments.

[t-bast]

Thanks, a last round of comments and we should be good to go.

[ysangkok]

@t-bast Ok, I have applied the suggestions and squashed.

[t-bast]

LGTM