Explicate travis ci setup

[k0pernikus]

In the contributing document it tells one to:

1. Enable Travis on your DT fork and test online.
   • See note below on how to enable this.
   • This takes a few steps to do but is faster for big cascading changes (like changing JQuery)
2. Depend on the automated test on Travis that runs after you send a PR.
   • If you need many attempts to fix errors then please flatten the history and clean the commit message.

As a wanna-be-definitelyTyped-contributor

• I might not know what travis ci is. A bit of context would be helpful, be it just a link to the project's run output
• I also don't know where the "note below" is and searching for travis inside the document I only see its occurrence in that quoted part, I do not see anything below related to it.
• I don't feel that the provided information suffice to setup the travis integration on one's own fork without further manual investigation

Instead, after reading this I rather feel like to just stop bothering right there and setup my custom typings instead of contributing to definitelyTyped project. While I don't expect handholding all they way, I prefer for a guide to offer a step-by-step information that keeps all the information in one place without linking "notes below" that one has to look for manually that I can just work through without it assuming I know all the technology and steps necessary to proceed further.

Hence, I kindly request the travis ci setup step to be explicated more.