diff --git a/ci/linkcheck.sh b/ci/linkcheck.sh
index 85573314..7149ab3a 100755
--- a/ci/linkcheck.sh
+++ b/ci/linkcheck.sh
@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
 
+# https://docs.travis-ci.com/user/environment-variables/#default-environment-variables
 if [ "$TRAVIS_EVENT_TYPE" = "cron" ] ; then # running in cron job
   FLAGS=""