Create a blog post sharing our plans for the year

[jpkrohling]

The blog post would contain both the items we are confident we can deliver, as well as the items we'd love to get help with.

[svrnm]

I created a draft PR, most content is still missing, but I will try to anticipate as much content as possible already, so we can publish it as soon as we have finalized the roadmap: open-telemetry/opentelemetry.io#3852