Spread the word about the new Slack channel for new contributors

[mx-psi]

After completing #53, we want to spread the word about the new Slack channel for contributors.

Here are some ways we can do this:

• When joining other Otel dev Slack channels (e.g. dev channels like #otel-arrow-dev, #otel-collector-dev, #otel-js-dev and any other channels that want to sign up) we can send a message to people telling them about this new channel
• Automations from different repositories that have a Github comment (example) can include a reference to this new channel
• The community repository and the opentelemetry.io website can mention it explicitly

[svrnm]

On oteldotio we can have a blog post and should have a page under https://opentelemetry.io/community/ for "how to get started contributing" or "new comers" or something, with some of the content we had in the past already in blog posts, a link to that channel, etc.