README.md: add cirrus-ci badge

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 3618079)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

README.md

@@ -5,6 +5,7 @@
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/588/badge)](https://bestpractices.coreinfrastructure.org/projects/588)
 [![gha/validate](https://github.com/opencontainers/runc/workflows/validate/badge.svg)](https://github.com/opencontainers/runc/actions?query=workflow%3Avalidate)
 [![gha/ci](https://github.com/opencontainers/runc/workflows/ci/badge.svg)](https://github.com/opencontainers/runc/actions?query=workflow%3Aci)
+[![CirrusCI](https://api.cirrus-ci.com/github/opencontainers/runc.svg)](https://cirrus-ci.com/github/opencontainers/runc)
 
 ## Introduction