docs: fix usage/readme.md to content of docs/index

Author: phanak-sap

docs/usage/README.md

@@ -4,6 +4,7 @@ The User Guide
 * [Initialization](initialization.rst)
 * [Querying](querying.rst)
 * [Creating](creating.rst)
+* [Updating](updating.rst)  
 * [Deleting](deleting.rst)
 * [Function Imports](function_imports.rst)
 * [Metadata](metadata.rst)