Modules documentation

Going through the getting started guide on modules explains the consul module file and goes right into the `apply` command. This gives an error because in the module docs it mentions the `get` command must be run to retrieve the modules first. I just pulled that same verbiage from `website/source/docs/modules/usage.html.md` and placed it on the `website/source/intro/getting-started/modules.html.md` page right before the apply section

## Changes

* Re-iterate 'terraform get' in getting started for modules