Add codspeed badge to README.rst (#1095)

We have been using codspeed a lot on this repo recently. We should add
it to the README as done on other projects to its easy to find, and
thank codspeed for offering their tooling for free to OSS.

Author: bdraco

README.rst

@@ -18,6 +18,10 @@ multidict
    :target: https://multidict.aio-libs.org
    :alt: Read The Docs build status badge
 
+.. image:: https://img.shields.io/endpoint?url=https://codspeed.io/badge.json
+   :target: https://codspeed.io/aio-libs/multidict
+   :alt: CodSpeed
+
 .. image:: https://img.shields.io/pypi/pyversions/multidict.svg
    :target: https://pypi.org/project/multidict
    :alt: Python versions