Set a default temporary and public file paths

From https://pantheon.io/docs/tmp#default-temporary-path, we should add something like this to settings.php:

```
/**
 * Define appropriate location for tmp directory
 */
if (isset($_ENV['PANTHEON_ENVIRONMENT'])) {
  $conf['file_temporary_path'] = $_SERVER['HOME'] .'/tmp';
}
```

And should do the same for public files and show how to to do it for private file directory.