Document the new security:check command

[javiereguiluz]

Symfony 2.6 will include a new security:check command to check your dependencies for the known security vulnerabilities: sensiolabs/SensioDistributionBundle#149

Should we include a reference of this command where appropriate in the Book or Cookbook? Should we include it just in the bundle's documentation?

[wouterj]

I think it'll be nice if we have a small .. note:: directive in book/security, but apart from that I think the bundle's docs are a perfect place.

[xabbuh]

If I remember correctly, we have some places where we describe a change in the behaviour due to security fixes (at least the maximum number of iterations in the password encoder). I think it makes sense to link to the security checker and introduce the security:check command there too.