Add more links to Slack + Introduce template chooser

[spier]

Let's make it easier for visitors of our patterns repo to find our slack.

To do so I did this:

• Add links to slack + book when creating an issue.
• Add link to Slack at the top of the main README.

As part of this I also created the base for the template chooser that GitHub offers.

We don't have templates yet but I am consider to add some templates e.g. for

• creating a quick draft of a new pattern
• submitting a Known Instance of a new pattern

Btw I got inspired to do this work when seeing the template chooser in the mermaid-js repo.

[spier]

I will just merge this to see it in action in the repo.

Then we can figure out later which issue templates may make sense.