Add security guidelines

[terhorstd]

This file defines the NEST security infrastructure. As discussed in the NEST-Initative Board.

The information will be available also in the project's Security Advisories page on GitHib (see next to Wiki above)

[jougs]

Fine with me, once my inline comments are addressed.

[Silmathoron]

Out of curiosity, what kind of security issues are we talking about here?

[terhorstd]

I think many things can be considered security related bugs. Basically anything that raises concerns of the user or machine/network admins. Main point is to define a way of contacting the developers on a secure channel. If a user would find a way to do any mischief with the simulator, it wouldn't be a good idea to put that into normal issue tracking or onto the mailing list. This of course assumes (a) something like this is possible, however unlikely, and (b) that it is a person is not tying to be next years supervillain.

[jougs]

Thanks for accepting my suggestions. I think this is better than having nothing and we always address further changes in future PRs. I'm thus merging without waiting for more reviews.

[heplesser]

@terhorstd Good work!