add Security team

[minrk]

document security role and add @rpwagner, @dlqqq. @rpwagner can you confirm both of these memberships, and can both of you confirm the info in the membership list?

I've created the @jupyterhub/security team with Security Manager permissions, what remains is to add members.

closes #720
closes #716

[willingc]

Thanks @minrk

[dlqqq]

I can confirm that I would like to have my account added to the security manager role. The messaging proposed here looks good to me.

[willingc]

After @rpwagner reviews, let's go ahead and merge this PR. Thanks all.

[choldgraf]

This looks good to me, thanks for writing this up, it's helpful to see it documented like this.

[rpwagner]

Confirming my access, thanks for documenting this process. It will help as more subprojects add this role.

[minrk]

Thanks all! I've added @rpwagner and @dlqqq to the security team with Security Manager permissions