Add "How to cite" to readme

from https://github.com/nexB/scancode-toolkit/issues/54#issuecomment-130385056
by @retrography:

We can't include that kind of disclaimer in an academic paper. An academic citation follows a very specific format. I give you an example from the `statnet` package that I use regularly for my analysis:

> Handcock M, Hunter D, Butts C, Goodreau S, Krivitsky P, Bender-deMoll S and Morris M (2015). _statnet: Software Tools for the Statistical Analysis of Network Data_. [The Statnet Project](http://www.statnet.org).

You can also provide the bibliographic entry, so that the users can format the citation according to the outlet they publish in:

```
@Misc{,
author = {Mark S. Handcock and David R. Hunter and Carter T. Butts and Steven M. Goodreau and Pavel N. Krivitsky and Skye Bender-deMoll and Martina Morris},
title = {statnet: Software Tools for the Statistical Analysis of Network Data},
organization = {The Statnet Project (\url{http://www.statnet.org})},
year = {2015},
note = {R package version 2015.6.2},
url = {CRAN.R-project.org/package=statnet},
}
```

Have a look at here: https://en.wikipedia.org/wiki/BibTeX