[#559] Add Tree service docs

Signed-off-by: Angira Kekteeva <kira@nspcc.ru>

README.md

@@ -88,6 +88,7 @@ AmazonS3 s3Client = AmazonS3ClientBuilder.standard()
 
 - [Configuration](./docs/configuration.md)
 - [NeoFS S3 AuthMate](./docs/authmate.md)
+- [NeoFS Tree service](./docs/tree_service.md)
 - [AWS CLI basic usage](./docs/aws_cli.md)
 - [AWS S3 API compatibility](./docs/aws_s3_compat.md)
 - [AWS S3 Compatibility test results](./docs/s3_test_results.md)

docs/tree_service.md

@@ -0,0 +1,18 @@
+# Tree service
+
+To get objects' metadata and system information, the S3 GW makes requests to the Tree service. 
+This is a service in NeoFS storage that keeps different information as a tree structure. 
+
+Each node keeps one of the types of data as a set of **key-value pairs**:
+* Bucket settings: lock configuration and versioning mode 
+* Bucket tagging
+* Object tagging
+* Object metadata: OID, name, creation time, system metadata
+* Object locking settings
+* Active multipart upload info
+
+Some data takes up a lot of memory, so we store it in NeoFS nodes as an object with payload. 
+But we keep these objects' metadata in the Tree service too:
+* Notification configuration
+* CORS
+* Metadata of parts of active multipart uploads