Documentation: Link for Koin Channel and Twitter

[Fuhrmann]

Describe the bug
There are 3 dead links I've found: one in the README which points to Koin channel (https://kotlinlang.org/messages/koin), one in the footer section of the documentation to your twitter page, and the Slack icon (in the docs, header) points to another link (https://slack.kotlinlang.org/messages/koin) that does not seems to work, I get this message:

Cannot GET /messages/koin

To Reproduce
Steps to reproduce the behavior:

1. Go to 'https://github.com/Ekito/koin'
2. Click on 'Come on slack Koin channel from Kotlin slack'
3. See error

Expected behavior
Should redirect to the pages.

Additional context
Sorry, I could not found the source code for the documentation so I did not send a pull request.
And the slack channel I don't know the correct URL.

Thanks.

[arnaudgiuliani]

Thanks for reporting the problem. We can't just provide the url as usual :/

[arnaudgiuliani]

Fixed :)

[Fuhrmann]

Nice, thanks!