remove duplicate directory in config_file_path

When trying to add custom tunnel clients, I came accross two errors:

1) I found out that the file that cf uses to check for custom clients is not `clients.yml` but `tunnel-clients.yml`. So I updated the readme accordingly. Can be verified [here](https://github.com/cloudfoundry/cf/blob/master/lib/tunnel/plugin.rb#L6)

2) I found a duplication in the `config_file_path` method. `CF::CONFIG_DIR` returns `.cf` as can be verified [here](https://github.com/cloudfoundry/cf/blob/master/lib/cf/constants.rb#L2). I removed the hardcoded `.cf` from the path.