Add a note about reporting security issues

[allait]

Since @kmike created a mailing list for security issues we should direct people there from the documentation.
Wording taken mostly from the Django docs: https://docs.djangoproject.com/en/dev/internals/contributing/bugs-and-features/#reporting-security-issues

[kmike]

+1. It may be a good idea to add this info to https://github.com/scrapy/scrapy.github.io as well.

[allait]

@kmike http://scrapy.org/community/ links to this documentation page as a place to start

[kmike]

Yes, but it also links to github repo and to scrapy-users mailing lists, and raising issue on github or sending message to a scrapy-users mailling list is natural enough - not everyone reads contributing guides.