adding an Introduction to the README

[mfrasca]

I'm trying to describe my understanding of the logic, structure and usage of this plugin, hoping it will help others, but also so that next time I have to use it, I will find the readme I missed the first time I came here.

[jsf-clabot]

All committers have signed the CLA.

[Arkni]

Thanks for your contribution! I'll take a look at it as soon as possible!

[Arkni]

Hi @mfrasca
Finally got some time to review this PR.

Lets start with some information about how the documentation is handled in grunt-contrib-* plugins.
All of the grunt-contrib-* plugins use grunt-contrib-internal to construct the README.md and CONTRIBUTING.md files. The source files are located in the corresponding docs/ folder.

Which means, you need to edit the source files rather than the README directly.

Your changes can be summarized in the following steps:

1. The introduction should be added in top of the file docs/qunit-overview.md alongside the other changes about configuring the task and the installation of PhantomJS.
2. The options updates should happen in the file docs/qunit-options.md, but please don't change the type of httpBase to Boolean. In fact, it should be a String and the default value should be changed to empty string "", which I will do after merging this PR.
3. Run grunt to generate the new README.md file and commit all files

Thanks.

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Note:
If your editor removes trilling white spaces from markdown files (especially the ones we use at the end of the type of every option), then replace them with <br>.

[mfrasca]

ok. seen. will make time to contribute my edits the way you indicate. :-)

[Arkni]

I removed one unrelated paragraph (the one about coveralls) and squashed the commits.

Thanks again for your contribution!