Add docs for builds based on docker

[notaphplover]

Hi, I've been using this library for sending a coverage report from a dockerized build hosted in Travis-CI.

At first I was unable to understand why coveralls.io builds had a coverage report but not a branch name nor a commit msg. At the end I had to read the library's code and this article to understand that node-coveralls needed some magic env variables from Travis in order to know the current branch.

I'm here just to suggest to add a few lines to the README.md file in order to explain that code coverage reports obtained from dockerized builds could need this env variables in order to use this library more easily :)