Document Appsignal::Config object
Document Appsignal::Config object

The new `config_file` param made me realize we haven't documented the
`Appsignal::Config` initializer, so I'm adding it now.

appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby