Add source link for Go taskfile variables

Some of the tasks for Go-related templates rely on taskfile variables, which are provided by the "asset" taskfile. The
`DEFAULT_GO_PACKAGES` variable is defined via some relatively complex code.

Documenting its source will facilitate project maintainers pulling in changes from development in this upstream
repository, and also pushing back upstream fixes or enhancements that are made in the downstream project.

Author: per1234

workflow-templates/assets/go-task/Taskfile.yml

@@ -2,6 +2,7 @@
 version: "3"
 
 vars:
+  # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/go-task/Taskfile.yml
   # Path of the project's primary Go module:
   DEFAULT_GO_MODULE_PATH: ./
   DEFAULT_GO_PACKAGES: