Tutorial for contribution

See https://github.com/ProjetPP/PPP-QuestionParsing-Grammatical/issues/144

We do not explain how to contribute to the existing modules. For instance, if one wants to add the wikidata property [P131](http://www.wikidata.org/wiki/Property:P131) to questions like `Where is the Eiffel tower`, he/she might not know where to add it.
