Introduce new PRIVACY.md document

A few days ago a discussion took place in IRC around whether or not we should consider adding Open Graph meta tags by default to be output on the front-end of a Ghost blog. The argument centered around privacy. While the OG tags would provide be of use and value to a large majority of users, there would also be privacy-concerned users who would not realise that this functionality was switched on by default.

The key point -- which was a good one -- was that unless you happen to view the correct source code at the correct time, it's impossible to know if and when things like this are occurring. Which kind of sucks.

Last year, a small movement started in the Open Source community which is slowly evolving into a standard practice: Including a SECURITY.md file in every repository - outlining the security policy for the project.

So, my idea is this: 

**PRIVACY.md- A plain English summary of all components within a repo which may affect user privacy. A single, standardised location to find out what's going on under the hood.** 

This PR is speculative and open to discussion. 

While we already have a flag to disable Ghost update-checks, I would suggest that a nice followup to this would be if there was a config flag for *every item* in PRIVACY.md to allow it to be disabled, if so desired. Along with a blanket rule flag, to disable absolutely everything. (Perhaps `tinfoil: true`)