[feature] Document how to use GoReleaser variables

[gabibguti]

Is your feature request related to a problem? Please describe.
I did not understand how to use variables such as {{ .Env.VERSION }} in SLSA GoReleaser configuration file. It was not clear to me if I need to define these variables somewhere using the suggested code, for example $(git describe --tags --always --dirty | cut -c2-) or $(echo $GITHUB_REF_NAME | cut -c2-) for {{ .Env.VERSION }}, or if they are ready to use as {{ .Env.VERSION }}. Since using directly {{ .Env.VERSION }} did not work for me, I imagine some other setup is needed, so improving the documented example usage would help.

Describe the solution you'd like
Document how to use GoReleaser variables such as {{ .Env.VERSION }} or improve the doc to show more example usages or needed setup.

Describe alternatives you've considered
None.

Additional context
None.