TASK: Update docs with installation instructions

The hard dependency to the Flowpack.ElasticSearch.ContentRepositoryAdaptor has been removed. Now you have to install the corresponing adaptor on your own. The new installation instructions shall call attention and provide guidance.

Author: dlubitz

README.md

@@ -1,9 +1,21 @@
 # Flowpack.SearchPlugin
 
 This plugin is a Search Plugin, to be used together with
-[Flowpack.ElasticSearch.ContentRepositoryAdaptor](https://github.com/Flowpack/Flowpack.ElasticSearch.ContentRepositoryAdaptor).
+* [Flowpack.ElasticSearch.ContentRepositoryAdaptor](https://github.com/Flowpack/Flowpack.ElasticSearch.ContentRepositoryAdaptor) or 
+* [Flowpack.SimpleSearch.ContentRepositoryAdaptor](https://github.com/Flowpack/Flowpack.SimpleSearch.ContentRepositoryAdaptor).
 
 ## Installation
+Install via composer with your favorite adaptor:
+
+**ElasticSearch**
+```
+composer require flowpack/searchplugin flowpack/elasticsearch-contentrepositoryadaptor
+```
+
+**SimpleSearch**
+```
+composer require flowpack/searchplugin flowpack/simplesearch-contentrepositoryadaptor
+```
 
 Inclusion of the routes from this package into your main `Configuration/Routes.yaml` is no longer needed as of Flow 4.0.