add download stats badge.
<a href="https://github.com/laixintao/iredis/actions"><img src="https://github.com/laixintao/iredis/workflows/Test/badge.svg" alt="Github Action"></a>
<a href="https://badge.fury.io/py/iredis"><img src="https://badge.fury.io/py/iredis.svg" alt="PyPI version"></a>
<img src="https://badgen.net/badge/python/3.6%20|%203.7%20|%203.8/" alt="Python version">
+<a href="https://pepy.tech/project/iredis"><img src="https://pepy.tech/badge/iredis" alt="Download stats"></a>
<a href="https://t.me/iredis_users"><img src="https://badgen.net/badge/icon/join?icon=telegram&amp;label=usergroup" alt="Chat on telegram"></a>
<a href="https://console.cloud.google.com/cloudshell/editor?cloudshell_git_repo=https://github.com/laixintao/iredis&amp;cloudshell_print=docs/cloudshell/run-in-docker.txt"><img src="https://badgen.net/badge/run/GoogleCloudShell/blue?icon=terminal" alt="Open in Cloud Shell"></a>
</p>
