Update ports.md

Author: probonopd

developer/ports.md

@@ -302,3 +302,7 @@ Result:
 * https://reviews.freebsd.org/D37380
 
 If a reviewer requires changes to be made, make the changes and upload (paste) a new diff (like the original diff; not a "diff of the diff"!) by clicking on "Update Diff" on the respective Phabricator page, e.g., https://reviews.freebsd.org/D37378.
+
+## Working with Cirrus CI
+
+To build packages on Cirrus CI, a `.cirrus.yml` file similar to https://github.com/b-aaz/xlibre-ports/blob/master/.cirrus.yml can be used.