Expand README

Author: chennes

README.md

@@ -1,19 +1,36 @@
 # FreeCAD Addon Manager
 
-This module was originally developed within FreeCAD, and has now been extracted into its own git repository. It is re-integrated into
-FreeCAD's source tree as a git submodule.
+Install and update third-party addons to FreeCAD, including Workbenches, Macros, Preference Packs, and more. Install
+*this* addon to update the internal Addon Manager to the latest version (and to allow future self-updating).
+
+This module was originally developed within FreeCAD, and has now been extracted into its own git repository. It is
+currently re-integrated into FreeCAD's source tree as a git submodule and continues to ship with FreeCAD.
+
+## Addon Sources
+
+The main source of addons is the git repository at https://github.com/FreeCAD/FreeCAD-Addons. Custom addon sources can
+be configured in the Addon Manager preferences when running FreeCAD. These addons are primarily written by
+third parties and provided by repositories not under the FreeCAD authors' or maintainers' control: you use these addons
+at your own risk.
+
+## Addon Manager Design Goals
+
+The Addon Manager is now designed to be self-updating, with a goal of allowing versions of FreeCAD back to 0.21 to
+use their default copy of Addon Manager to install a new version of Addon Manager. This means that the Addon Manager
+should support PySide2 and Python 3.8 for the foreseeable future.
+
+The Addon Manager is also designed to be run in a "standalone" mode to allow for easier UI development. In this mode
+it does not interact with FreeCAD at all, and does not use or affect "real" FreeCAD preferences, module installation,
+etc.
 
 ## Roadmap
 
 This module is under active development, with the following rough plan
 
-1. Complete migration to using the FreeCAD wrapper class to allow running outside of FreeCAD.
-   1. Split `addonmanager_utilities.py` along GUI/No-GUI lines
-   2. Refactor tests into pure GUI/No-GUI lines
-   3. Verify that all `FreeCAD.*` and `FreeCADGui.*` calls are wrapped
-2. Update GitHub CI to run Addon Manager test suite.
-3. Migrate to a JSON-formatted addon repository list.
-4. Rearrange codebase to better separate GUI from logic code.
+1. Migrate to a JSON-formatted addon repository catalog, replacing the original `.gitmodules`-based addon source
+2. Rearrange codebase to better separate GUI from logic code.
+3. Implement detection of changed addon dependencies when updating.
+4. Implement remote caching of icons and macros.
 5. Begin GUI redesign.
 
 PRs are welcome!