Add .env to optionally ignored files

Many twelve-factor apps use environment variables rather than the built in Rails secrets mechanism to store secrets.
The [dotenv](https://github.com/bkeepers/dotenv) gem is widely used for this purpose and allows environment variables to be loaded from the `.env` file. However, this file should not be committed to source control.

This PR just adds the `.env` file to `.gitignore` so that these secrets aren't accidentally committed. In some rare instances, it may be required that this file is committed to source control so I've added it to the list of files that can be removed if the developer is ok with secrets being committed.