docs: Add note to manually publish an image

Author: akash1810

.env

@@ -2,6 +2,8 @@
 
 # Environment variables shared between ci and DEV.
 
+# NOTE: If you change this value remember to produce a new image by running the "CQ image" workflow.
+# See https://github.com/guardian/service-catalogue/actions/workflows/cq-image.yml
 # See https://github.com/cloudquery/cloudquery/releases?q=cli
 CQ_CLI=6.15.0