Add note about buf-push-action and workspaces

[amckinney]

We've had a couple conversations around how someone would use buf-push-action in a repository with multiple modules. Ideally, all of this would just work for workspaces (see this issue), but in the meantime it's still useful to have some guidance: simply specify the action more than once.

[marekbuild]

Thanks for adding this, which will hopefully make it more clear that you can do it, and it's just a matter of specifying multiple pushes.