yarn-plugin: support only "backstage:^"

Signed-off-by: MT Lewis <mtlewis@users.noreply.github.com>

packages/yarn-plugin/README.md

@@ -1,12 +1,13 @@
 # yarn-plugin-backstage
 
 This yarn plugin adds a `backstage:` version protocol to yarn, which replaces
-specific version ranges for `@backstage/` packages. The recommended mode of use
-is to set all versions strings to `backstage:*` in package.json, which causes
-all versions to be resolved based on the version of Backstage specified in
-`backstage.json`. This ensures that all `@backstage/` packages always correspond
-to a single release, and removes the need for `package.json` files
-to change when upgrading to a new release.
+specific version ranges for `@backstage/` packages. The only version range
+supported by this plugin is `backstage:^`, which has similar semantics to the
+corresponding [`workspace` range](https://yarnpkg.com/features/workspaces#cross-references) when using
+workspace dependencies; locally, the package will always resolve to the exact
+version specified in the manifest for the Backstage release listed in
+backstage.json. If the dependent package is published, this version will be
+prefixed by `^`.
 
 **This plugin is still under active development, and requires some further
 testing before we recommend it for general use.**