Add codspeed badge (aio-libs#9751)

Author: bdraco

README.rst

@@ -17,6 +17,10 @@ Async http client/server framework
    :target: https://codecov.io/gh/aio-libs/aiohttp
    :alt: codecov.io status for master branch
 
+.. image:: https://img.shields.io/endpoint?url=https://codspeed.io/badge.json
+   :target: https://codspeed.io/aio-libs/aiohttp
+   :alt: Codspeed.io status for aiohttp
+
 .. image:: https://badge.fury.io/py/aiohttp.svg
    :target: https://pypi.org/project/aiohttp
    :alt: Latest PyPI package version