Announce 1.0.0 (WIP)

Add 1.0.0 announcment document and point to it in main README.

TODO:
- Commit message
- PR (blocks on theupdateframework#1693, theupdateframework#1675, maybe theupdateframework#1700)

1.0.0-ANNOUNCEMENT.md

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+# Announcing TUF 1.0.0
+
+In the past year we have made an effort to revise, redesign and rewrite this
+python-tuf reference implementation, and are very excited to announce a stable
+1.0.0 release. The release *will* include:
+- a modern low-level [Metadata
+  API](https://theupdateframework.readthedocs.io/en/latest/api/tuf.api.html)
+- a fully specification-compliant [updater
+  client](https://theupdateframework.readthedocs.io/en/latest/api/tuf.ngclient.html),
+  serving as a more robust and yet more flexible stand-in replacement
+  for the legacy client updater
+
+For the reasons outlined in [ADR 10](docs/adr/0010-repository-library-design.md
+), this release *will not* include a new repository tool. That said, the new
+Metadata API now makes it easy to replicate the desired functionality based on
+the specific needs of a given repository (more about *migration* below).
+
+As discussed in [ADR 2](docs/adr/0002-pre-1-0-deprecation-strategy.md), this
+release *will not* include any legacy code, as its maintenance has become
+infeasible for the python-tuf team. The pre-1.0.0 deprecation strategy from ADR
+2 reads as follows:
+
+> *Bugs reported with tuf versions prior to 1.0.0 will likely not be addressed
+directly by tuf’s maintainers. Pull Requests to fix bugs in the last release
+prior to 1.0.0 will be considered, and merged (subject to normal review
+processes). Note that there may be delays due to the lack of developer resources
+for reviewing such pull requests.*
+
+
+## Migration
+
+Given the clean cut with the legacy reference implementation, we provide the
+following migration support:
+
+- detailed code documentation on [theupdateframework.readthedocs.io](https://theupdateframework.readthedocs.io/)
+- verbose [code examples](examples/) for client updater usage, and
+  repository-side operations based on the low-level metadata API
+- individual migration support upon [request](https://github.com/theupdateframework/python-tuf#contact)
+- targeted migration support for known users

README.md

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 [![CII](https://bestpractices.coreinfrastructure.org/projects/1351/badge)](https://bestpractices.coreinfrastructure.org/projects/1351)
 [![PyPI](https://img.shields.io/pypi/v/tuf)](https://pypi.org/project/tuf/)
 
+----------------------------
+*__IMPORTANT NOTICE:__ A backwards-incompatible 1.0.0 release is scheduled for
+January 2022. Please see [this *1.0.0 announcement*](1.0.0-ANNOUNCEMENT.md)
+document for more details about the release, legacy code maintenance promises,
+and migration instructions.*
+
 ----------------------------
 This repository is the **reference implementation** of
 [The Update Framework (TUF)](https://theupdateframework.github.io/).