Remove Slack link and other updates to README

[tnull]

This started off as me simply wanting to remove the deprecated Slack link, but I couldn't keep myself from making at least some additional changes while touching the README.

I think it could be updated quite a bit more, but these changes are maybe a first step.

[codecov-commenter]

Codecov Report

Merging #1628 (16d40ef) into main (834fe63) will decrease coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 16d40ef differs from pull request most recent head cf7655a. Consider uploading reports for the commit cf7655a to get more accurate results

@@            Coverage Diff             @@
##             main    #1628      +/-   ##
==========================================
- Coverage   90.80%   90.78%   -0.03%     
==========================================
  Files          80       80              
  Lines       44654    44654              
  Branches    44654    44654              
==========================================
- Hits        40550    40539      -11     
- Misses       4104     4115      +11     

Impacted Files	Coverage Δ
lightning/src/ln/functional_tests.rs	96.87% <0.00%> (-0.17%)	⬇️

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[dunxen]

Suggested change

	Implements utilities to manage Rust-Lightning channel data persistence and retrieval.
	Implements utilities to manage rust-lightning channel data persistence and retrieval.

Repo name should probably be lowercase for consistency too (unless start of a sentence)? Here and elsewhere there's a mix of "Rust-Lightning" and "rust-lightning"

[tnull]

Mh, I'm not sure if there is consensus on this. I'd say if we see Rust-Lightning as a thing that is more than the main crates itself, it probably should be capitalized. However, I'd argue rust-lightning is just the Rust library and the 'more' part would be LDK and therefore agree to keep it consistently lower case. So I now replaced Rust-Lightning with rust-lightning in most places, except for headings and the tag line.

[dunxen]

Ok, yeah that makes sense. I guess Rust-Lightning was in its previous life :)

[wpaulino]

Is there a reason for reverting this?

[tnull]

Yes, I think rust-lightning in headings and/or the tag line looks odd and decided to revert to title case for these, i.e., Rust-Lightning. Happy to always use rust-lightning if there is a clear preference for that though.

[wpaulino]

I prefer keeping it consistent, no big deal though.

[wpaulino]

LGTM

[TheBlueMatt]

I think feel free to squash.

[tnull]

Squashed