message: Add CircleCI badge to manifest reference
  rust-lang/crates.io#807
diff: @@ -163,6 +163,8 @@ travis-ci = { repository = "...", branch = "master" }
  appveyor = { repository = "...", branch = "master", service = "github" }
  # GitLab: `repository` is required. `branch` is optional; default is `master`
  gitlab = { repository = "...", branch = "master" }
  +# Circle CI: `repository` is required. `branch` is optiona; default is `master`
  +circle-ci = { repository = "...", branch = "master" }
  # Is it maintained resolution time: `repository` is required.
  is-it-maintained-issue-resolution = { repository = "..." }
  # Is it maintained percentage of open issues: `repository` is required.
  