[docs]Describe how to create and use finder

doc/finder.md

@@ -0,0 +1,35 @@
+# Finder
+
+The service that enables you to fetch objects from Elasticsearch is called `Finder`.
+You can create a custom Finder by implementing the `FinderInterface` interface and map it in config or via Symfony a tag.
+
+Most projects require pagination. To enable paginationt - implement the `PaginatedFinderInterface`.
+
+## Via Symfony tags
+```yaml
+# config/services.yaml
+App\Finder\UserFinder:
+    class: App\Finder\UserFinder
+    tags:
+        - { name: fos_elastica.finder, index: user }
+```
+
+
+## Via config yaml file
+
+```yaml
+# config/services.yaml
+App\Finder\UserFinder:
+    class: App\Finder\UserFinder
+```
+
+```yaml
+# config/packages/fos_elastica.yaml
+fos_elastica:
+    indexes:
+        user: # Index Name
+            persistence:
+                finder:
+                    service: App\Finder\UserFinder
+
+```