Update README with command line params

[JonathanPorta]

I was having an issue where node-coveralls was assuming the filepath in the LCOV report is relative to the current working directory. This was causing a file not found exception in lib/ConvertLcovToCoveralls.js. Looking through the node-coveralls code, I figured out that I could specify the base file path as a command line argument. [1]

Could you please document this in the README? I am not sure which other command line params are supported, otherwise I would submit a PR. ;)

[1] https://github.com/cainus/node-coveralls/blob/master/lib/getOptions.js#L109