Update @plone/* packages info (#6521)

Co-authored-by: Steve Piercy <[email protected]>

PACKAGES.md

@@ -2,17 +2,37 @@
 
 This document describes the packages that come with Volto, the default frontend for Plone 6.
 
+These packages are part of Plone's API-first story.
+Most of them are experimental and are marked in their respective `README` files.
+Plone 6.1.x (Volto 18) depends on:
+- `@plone/registry`
+- `@plone/scripts`
+- `@plone/volto-slate`
+
+and as a development dependency:
+- `@plone/types`
+
+Plone 6.0.x (Volto 17 and below) does not use any of them.
+
+These packages are expected to be used and become part of Plone 7.
+Some of them might become part of Plone 6.1.x minor versions.
+
+The packages are divided into three categories or types:
+
+- core
+- utilities
+- add-ons
+
 
 ## `@plone/types`
 
 Plone types is a special development package.
 It contains the Plone typings for TypeScript.
-It's considered a core package, and it's the only package that the other core packages can rely on as
-a `devDependency` in your project configuration.
+It's considered a core package, and it's the only package that the other core packages can rely on as a `devDependency` in your project configuration.
 
 This package contains `.d.ts` typing definitions, curated by hand.
-Due to the nature of this package, it does not need bundling.
-It's published "as is", so you can import the type definitions from anywhere in your code.
+Due to the nature of this package, it does not need to be built nor bundled.
+It is published "as is", so you can import the type definitions from anywhere in your code.
 
 
 ## Core packages
@@ -29,12 +49,10 @@ They must be published and bundled in a traditional (transpiled) way.
 The bundle of these packages must work on both CommonJS and ECMAScript Module (ESM) environments.
 
 
-## Utility packages
+## Utilities packages
 
--   `@plone/drivers`
--   `@plone/helpers`
 -   `@plone/providers`
--   `@plone/rsc`
+-   `@plone/helpers`
 
 
 ### Rules
@@ -44,19 +62,22 @@ They must be published in the traditional way, as a bundle.
 This bundle must work on both CommonJS and ESM environments.
 
 
-## Feature packages
+## Add-on packages
 
 -   `@plone/blocks`
--   `@plone/contents`
 -   `@plone/slots`
+-   `@plone/theming`
+-   `@plone/contents`
 
 
 ### Rules
 
-Feature packages, or add-on packages, can depend on any other package.
+Add-on or feature packages, can depend on any other package.
 You must distribute them as source code, and not transpile them.
 They must provide a default configuration registry loader as the default main entry point export.
-They must be loadable as any other add-on.
+Unlike Volto add-ons, do *NOT* place the code in the `src` folder.
+If you do not transpile the package, the direct resolution must work out of the box, where both the bundler and TypeScript resolution are direct.
+They must be loadable as any other add-on, and contain an add-on registry compatible `install`-able default export.
 
 
 ## Development utility packages