Updated README/docs with notes about the v2 changes

docs/en/changes-in-v2.md

@@ -0,0 +1,104 @@
+# Changes in Version 2
+
+## Preamble
+
+If your existing implementation of this module simply uses the out of the box extensions, jobs, and tasks, without
+any customisations, then it's very likely that the changes to Environment Variable names would be the only change
+required for you.
+
+## Breaking changes
+
+### PHP
+
+Minimum PHP requirement has been increased to PHP 8.
+
+### Environment variables
+
+Usages of the name `APP` has been replaced with `ENTERPRISE`.
+
+Before:
+```yml
+APP_SEARCH_ENGINE_PREFIX=""
+APP_SEARCH_API_KEY=""
+APP_SEARCH_API_SEARCH_KEY=""
+APP_SEARCH_ENDPOINT=""
+```
+
+After:
+```yml
+ENTERPRISE_SEARCH_ENGINE_PREFIX=""
+ENTERPRISE_SEARCH_API_KEY=""
+ENTERPRISE_SEARCH_API_SEARCH_KEY=""
+ENTERPRISE_SEARCH_ENDPOINT=""
+```
+
+### Method return types
+
+Some methods previously returned `void` or `$this`, which meant they didn't give any useful feedback from the request
+we had just made.
+
+Methods like `EnterpriseSearchService::configure()` now return info about the response (in this case, an array of the
+active configuration for each index).
+
+Methods with changed return types:
+
+* `IndexingInterface::configure()`
+* `IndexingInterface::addDocument()`
+* `IndexingInterface::removeDocument()`
+* `BatchDocumentInterface::addDocuments()`
+* `BatchDocumentInterface::removeDocuments()`
+
+### Pretty much every property/parameter everywhere
+
+I think we've been able to keep the general API for all methods (that being, the order and purpose of params) the same,
+but a lot of methods have had type declarations added, so if you are extended these classes and overriding methods,
+then these would likely be breaking changes for you.
+
+Where possible, properties have been given type hints (where they didn't previously). Exceptions are the usual
+properties that extend some other vendor classes that they have to match.
+
+### `NaiveSearchService`
+
+Practically, I don't think anyone would have been accessing this class, but the namespace has changed to be consistent
+with other classes.
+
+Before: `SilverStripe\SearchService\Services\Naive\NaiveSearchService`
+After: `SilverStripe\SearchService\Service\Naive\NaiveSearchService`
+
+## Maybe breaking
+
+### Default HTTP Client
+
+We **don't think** this will be a breaking change, as the old App Search already explicitly used `Guzzle` as its HTTP
+Client. However, the new Enterprise search uses PSR-18 "discovery". We found this to be a bit too fragile (EG: It was
+breaking with Symfony Client), so we've added a new config to the `ClientFactory` to allow folks to specify what client
+they would like instantiated. Default is `Guzzle`.
+
+```yml
+SilverStripe\Core\Injector\Injector:
+  Elastic\EnterpriseSearch\Client:
+    factory: SilverStripe\SearchService\Service\EnterpriseSearch\ClientFactory
+    constructor:
+      # original configs
+      host: '`ENTERPRISE_SEARCH_ENDPOINT`'
+      token: '`ENTERPRISE_SEARCH_API_KEY`'
+      # new config
+      http_client: '%$GuzzleHttp\Client'
+```
+
+The Elastic Enterprise dependency seems to be tested on Guzzle specifically though, so any change here would need to be
+covered by your own testing.
+
+### `recordBaseClass` and `pageContent`
+
+There were two conflicting configs, one defined these fields in snake case (EG: `record_base_class`), and the other
+defined them in camel case (EG: `recordBaseClass`). I've standardised all configs to snake case.
+
+Looking through some of my own projects, we seem to use the snake case, so this might not be a breaking change.
+
+## New features
+
+### New Permission for triggering ReIndex through Service Admin
+
+`SearchAdmin_ReIndex` permission has been added so that it's not longer only ADMIN who are able to trigger a full
+ReIndex through the Model Admin area.

docs/en/index.md

@@ -29,4 +29,5 @@
     * [Event handling](customising_more.md#event-handling)
     * [Document metadata](customising_more.md#document-meta)
     * [Extension points](customising_more.md#extension-points)
+* [Changes in v2](changes-in-v2.md)