Add .gitignore for generated projects

Hacking on a Hoplon project today, I noted the project generated with `lein new hoplon ...` did not include a `.gitignore` file. As a result, I almost checked in a large number of files that `boot` creates in `.boot/`, as well as the generated output (`resources/`).

Do you think it would be reasonable to create a `.gitignore` with those two folders?