Document what is required for a green build

[niccokunzmann]

We now have travis running #248. Goal:

• provide a steep learning curve for newcomers

• Create 'more easy issues' based on travis where newcomers can contribute

• Document what travis needs on order to be green.

• Add examples of what could go wrong and how to solve it. (There might be special error messages almost unclear to nonprogrammers)

[idealadarsh]

For a green build:

• Make sure the file is present where the link is pointing for it. e.g. There was a line src="style/bootstrap/js/jquery.min.js" but there was no jquery.min.js in style>bootstrap>js folder.
• Don't link to any external resource.