Mention existence of 5.x.x branch and v5.0.0-alpha.0 in README

[voxpelli]

As a follow up to my confusion in two recent PR:s (#1528 (comment), #1527 (review)) I want to suggest the README in the main branch to be updated to mention the existence of the 5.x.x branch + ideally the web page to mention the existence of 5.x.xand/or v5.0.0-alpha.0

On top of that it would be good to create a new 5.0.0 milestone and attach PR:s like the two mentioned above to that milestone (and maybe close the 4.0 milestone that's the current only milestone: https://github.com/chaijs/chai/milestones)

[keithamus]

Sounds like a good idea. Would you like to make a PR for the readme?

[voxpelli]

@keithamus I could, but not sure I would do it much justice as I can't really describe the 5.x.x effort in any wider fashion than the fact that it exists

[koddsson]

I'm gonna close this as we don't have a v5 branch anymore, it's just main.