prepare elastic update instructions

doc/update/8.7-to-8.8.md

@@ -137,6 +137,44 @@ To make sure you didn't miss anything run a more thorough check:
 
 If all items are green, then congratulations, the upgrade is complete!
 
+### 10. Elasticsearch indexes update (if you currently use Elasticsearch)
+
+When index mapping is changed the whole index should be removed and built from the scratch.
+
+1. Delete old indexes:
+
+    ```
+    # Omnibus installations
+    sudo gitlab-rake gitlab:elastic:delete_indexes
+
+    # Installations from source
+    bundle exec rake gitlab:elastic:delete_indexes
+    ```
+
+1. Create new, empty indexes:
+
+    ```
+    # Omnibus installations
+    sudo gitlab-rake gitlab:elastic:create_empty_indexes
+
+    # Installations from source
+    bundle exec rake gitlab:elastic:create_empty_indexes
+    ```
+
+1. Fill newly created indexes. The following command is acceptable for not very big GitLab instances (storage size no more than few gigabytes)
+
+    ```
+    # Omnibus installations
+    sudo gitlab-rake gitlab:elastic:index
+
+    # Installations from source
+    bundle exec rake gitlab:elastic:index
+    ```
+
+  If your instance is big we recommend to follow [Add GitLab's data to the Elasticsearch index](../integration/elasticsearch.md#add-gitlabs-data-to-the-elasticsearch-index)
+
+
+
 ## Things went south? Revert to previous version (8.6)
 
 ### 1. Revert the code to the previous version