Added badge for API reference (#495)

* Added badge for API reference

* Apply suggestions from code review

* Apply suggestions from code review

* Apply suggestions from code review

Author: asmyasnikov

README.md

@@ -2,6 +2,7 @@ YDB Python SDK
 ---
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://github.com/ydb-platform/ydb/blob/main/LICENSE)
 [![PyPI version](https://badge.fury.io/py/ydb.svg)](https://badge.fury.io/py/ydb)
+[![API Reference](https://img.shields.io/badge/API-Reference-lightgreen.svg)](https://ydb-platform.github.io/ydb-python-sdk)
 [![Functional tests](https://github.com/ydb-platform/ydb-python-sdk/actions/workflows/tests.yaml/badge.svg)](https://github.com/ydb-platform/ydb-python-sdk/actions/workflows/tests.yaml)
 [![Style checks](https://github.com/ydb-platform/ydb-python-sdk/actions/workflows/style.yaml/badge.svg)](https://github.com/ydb-platform/ydb-python-sdk/actions/workflows/style.yaml)