doc - explain why we forked and what are the differences

[severo]

I think we should explain why we forked the geonetwork project, and list the differences in the main README.md.

I will not fill other issues, but the same applies to: https://github.com/georchestra/geoserver, https://github.com/georchestra/geoext, https://github.com/georchestra/openlayers, https://github.com/georchestra/styler, https://github.com/georchestra/geotools, https://github.com/georchestra/extjs.

Note that I could not answer the question, and I want to know :)