README: describe the current state of the project refactor

Signed-off-by: Joshua Lock <jlock@vmware.com>

Author: joshuagl

README.md

@@ -10,9 +10,36 @@ This repository is the **reference implementation** of
 [The Update Framework (TUF)](https://theupdateframework.github.io/).
 It is written in Python and intended to conform to version 1.0 of the
 [TUF specification](https://theupdateframework.github.io/specification/latest/).
-This implementation is in use in production systems, but is also intended to be
-a readable guide and demonstration for those working on implementing TUF in
-their own languages, environments, or update systems.
+
+The repository currently includes two implementations:
+1) A *legacy implementation*, with
+   [`tuf/client/updater.py`](tuf/client/updater.py) implementing the detailed
+   client workflow and [`tuf/repository_tool.py`](tuf/repository_tool.py)
+   providing a high-level interface for repository operations.
+   The legacy implementation is in use in production systems, but is [no longer
+   being actively worked on](docs/adr/0002-pre-1-0-deprecation-strategy.md).
+2) A *modern implementation*. We are in the process of rewriting the reference
+   implementation in [modern Python](docs/adr/0001-python-version-3-6-plus.md)
+   to both: a) address scalability and integration issues identified in
+   supporting integration into the Python Package Index (PyPI), and other
+   large-scale repositories, and b) to ensure maintainability of the project.
+   This implementation consists of:
+   * a "low-level" metadata API, designed to provide easy and safe access to
+     TUF metadata and handle (de)serialization from/to files, provided in the
+     [`tuf/api/metadata.py`](tuf/api/metadata.py) module.
+   * an implementation of the detailed client workflow built on top of the
+     metadata API, provided in the
+     [`tuf/ngclient/updater.py`](tuf/ngclient/updater.py) module.
+   The modern implementation is not considered production ready and does not yet
+   provide any high-level support for implementing
+   [repository operations](https://theupdateframework.github.io/specification/latest/#repository-operations),
+   though the addition of API to support them is planned.
+
+
+
+The reference implementation strives to be a readable guide and demonstration
+for those working on implementing TUF in their own languages, environments, or
+update systems.
 
 
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