Clearly describe current project state and how to contribute

Update `README.md` and `CONTRIBUTING.md` to clearly describe the current state of the project and how best to contribute to it.

We want to encourage people to help get Pekko to v1.0, and ensure it's clear that this is being actively worked on. We don't want to discourage people with inaccurate docs, or by appearing to claim Pekko is ready for use now when it's not.

I'm keen to hear other folks' thoughts, but here are some things I think we could do to improve the situation:

* Near the top of the README, say "Pekko is part of the Apache Incubator program and we are actively working towards a v1.0 release. It's not yet ready for production use, but please get involved with the community and contribute to the effort."
* In CONTRIBUTING (or possibly even README), add a section explaining that we are working on `incubator-pekko` first and not touching any of the other repos. This is quite important and not immediately obvious.
* In CONTRIBUTING, remove (or demote) the section that lists how Akka used to use tags (labels). We aren't using these yet. If we are using labels etc, explain that instead.
* Agree on label/milestone usage: I can see we're using `milestone:1.1.0` for things that don't belong in the initial release; I think we should use `blocked` for issues that are blocked, e.g., #101. I wonder if there's a way we can label priorities or classes of issues to make the list more manageable?
* In CONTRIBUTING, add some kind of table of contents - otherwise it's not obvious that build instructions are down below.

I also think we should try to get the complete list of things needed to reach v1.0 raised as issues, rather than relying on memory and mailing lists. That way we have an accurate work list, and we can tell folks "check GitHub for the current list of open issues, read the discussion, and dive in".

Feedback welcome - I'm new to this project and am probably missing important context. Thanks!