doc: announce 1.0.0 stable release

* Add a document that announces a stable "modern
  implementation"-only 1.0.0 release, with the following contents:
  - the scheduled release date
  - contents of release (metadata API, ngclient, no legacy code)
  - legacy code deprecation note (adr 2)
  - note about lack of repository rool (adr 10)
  - migration instructions

* Add "important notice" to head of main README, pointing to above
  document.

Signed-off-by: Lukas Puehringer <[email protected]>

1.0.0-ANNOUNCEMENT.md

@@ -0,0 +1,43 @@
+# 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 we are very excited to announce a
+stable 1.0.0 release scheduled for January 2022. 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
+
+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 applies 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.*
+
+For the reasons outlined in [ADR 10](docs/adr/0010-repository-library-design.md
+), this release *will not* include a new *repository tool*. However, the new
+*metadata API* makes it easy to replicate the desired functionality tailored to
+the specific needs of a any given repository (see *Migration* for details).
+
+
+
+
+## Migration
+
+Given the clean cut with the legacy reference implementation, we provide the
+following migration support:
+
+- detailed code documentation on
+  [https://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 initiative for known users

README.md

@@ -6,6 +6,13 @@
 [![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 stable 1.0.0 release of the modern implementation only
+is scheduled for January 2022. Please see the [*1.0.0
+announcement*](1.0.0-ANNOUNCEMENT.md) page for more details about the release
+and the deprecation of the legacy implementation, including migration
+instructions.*
+
 ----------------------------
 This repository is the **reference implementation** of
 [The Update Framework (TUF)](https://theupdateframework.github.io/).