Document the 1.x / 2.x split and alias dev-main to 2.0.x-dev (#10)

Branch `1` now holds the CMS 5 line and `main` carries CMS 6 / Forager 2
support, tagged 2.0.0-alpha1. Add a branch-alias so existing dev-main
consumers are not silently moved onto the 2.x code, and a README table
explaining which line to install.

Author: blueo

README.md

@@ -25,8 +25,22 @@ interchangably in this module. This module does not currently provide support fo
 
 ## Installation
 
+### Which version?
+
+| Line | Branch and tags | Requires | Status |
+| --- | --- | --- | --- |
+| 1.x | branch `1`, tags `1.0.0`–`1.1.0` | Silverstripe CMS 5, Forager `^1.3`, PHP `^8.1` | Supported stable line |
+| 2.x | branch `main`, tag `2.0.0-alpha1` | Silverstripe CMS 6, Forager `^2.0`, PHP `^8.3` | Alpha release |
+
+Install the latest 1.x release for CMS5 with:
+
 `composer require silverstripe/silverstripe-forager-elastic-enterprise`
 
+The 2.x line adds CMS 6 and Forager 2 support. It is tagged as an alpha because the API may change with future updates.
+Composer will not resolve it unless you opt in explicitly:
+
+`composer require silverstripe/silverstripe-forager-elastic-enterprise:^2.0@alpha`
+
 ## Activating EnterpriseSearch
 
 To start using Elastic Enterprise Search, define environment variables containing your private API key, endpoint, and

composer.json

@@ -44,6 +44,11 @@
             "SilverStripe\\ForagerElasticEnterprise\\Tests\\": "tests/"
         }
     },
+    "extra": {
+        "branch-alias": {
+            "dev-main": "2.0.x-dev"
+        }
+    },
     "minimum-stability": "dev",
     "prefer-stable": true,
     "config": {