chore: mention KQL in the search readme

services/search/README.md

@@ -25,6 +25,22 @@ Note that as of now, the search service can not be scaled. Consider using a dedi
 
 By default, the search service is shipped with [bleve](https://github.com/blevesearch/bleve) as its primary search engine. The available engines can be extended by implementing the [Engine](pkg/engine/engine.go) interface and making that engine available.
 
+## Query language
+
+By default, [KQL](https://learn.microsoft.com/en-us/sharepoint/dev/general-development/keyword-query-language-kql-syntax-reference) is used as query language,
+for an overview of how the syntax works, please read the [microsoft documentation](https://learn.microsoft.com/en-us/sharepoint/dev/general-development/keyword-query-language-kql-syntax-reference).
+
+Not all parts are supported, the following list gives an overview of parts that are not implemented yet:
+
+* Synonym operators
+* Inclusion and exclusion operators
+* Dynamic ranking operator
+* ONEAR operator
+* NEAR operator
+* Date intervals
+
+In the following [ADR](https://github.com/owncloud/ocis/blob/docs/ocis/adr/0020-file-search-query-language.md) you can read why we chose KQL.
+
 ## Extraction Engines
 
 The search service provides the following extraction engines and their results are used as index for searching: