Slack invite link is broken

[KokoseiJ]

Slack invite link in README.md and CONTRIBUTING.md is broken, It should be replaced to new one.

[arcanearronax]

Is this project active? The link appears to still be broken.

[KokoseiJ]

Not that active, Author is busy, I'm too busy to add new features it, too.

I'm willing to accept any reasonable PRs though, so Feel free to send one, If you're planning to do it.