Add 1.0.0 announcement / migration help

[lukpueh]

Description of issue or feature request:

Add a document to the repo that announces the upcoming 1.0.0 release and provides help on how to migrate. The document should have at least the following contents:

• 1.0.0 release date -- ~early 2022 or more specific?
• summary of ADR 2 -- pre-1.0.0 code will remain available and be supported on a best-effort basis ( ADR 2 )
• link to client API docs and usage example (RTD, docs: tutorial/example for ngclient Updater #1518)
• link to metadata API docs and (repo) usage example (RTD, Docs: Add repository tutorial based on metadata API #1673)
• disclaimer about the temporary lack of repository tool in 1.0.0 -- consolation: we are working hard on a replacement, in the meanwhile you can use the metadata API (Design repository library #1136)
• migration help:
  • see usage examples
  • reach out to us, if you need help
  • we will also reach out to users that we know of

We could reference this document in some or all of the following channels:

• slack
• mailinglist
• release notes of a final pre-1.0.0 release
• readme
• website