Add references to Asciidoctor Chat (Zulip) to README and issue template

[abelsromero]

Thank you for opening a pull request and contributing to asciidoctor-maven-plugin!

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [x] Documentation
• [ ] Refactor
• [ ] Build improvement
• [ ] Other (please describe)

What is the goal of this pull request?

Help users with questions locate the chat site.

Are there any alternative ways to implement this?

no

Are there any implications of this pull request? Anything a user must know?

no

Is it related to an existing issue?

- [ ] Yes - [x] No *Finally, please add a corresponding entry to CHANGELOG.adoc*