Add Cirrus CI badge to manifest

Reflect changes from rust-lang/crates.io#1782.

Author: jonhoo

src/doc/src/reference/manifest.md

@@ -269,7 +269,7 @@ license-file = "..."
 # Optional specification of badges to be displayed on crates.io.
 #
 # - The badges pertaining to build status that are currently available are
-#   Appveyor, CircleCI, GitLab, Azure DevOps and TravisCI.
+#   Appveyor, CircleCI, Cirrus CI, GitLab, Azure DevOps and TravisCI.
 # - Available badges pertaining to code test coverage are Codecov and
 #   Coveralls.
 # - There are also maintenance-related badges based on isitmaintained.com
@@ -290,6 +290,9 @@ appveyor = { repository = "...", branch = "master", service = "github" }
 # Circle CI: `repository` is required. `branch` is optional; default is `master`
 circle-ci = { repository = "...", branch = "master" }
 
+# Cirrus CI: `repository` is required. `branch` is optional; default is `master`
+cirrus-ci = { repository = "...", branch = "master" }
+
 # GitLab: `repository` is required. `branch` is optional; default is `master`
 gitlab = { repository = "...", branch = "master" }