Create Security documentation

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There are:

Security policies

Without custom flags at installation, or subsequent interface connections, snaps are confined to a restrictive security sandbox with no access to system resources outside of the snap. Snap developers need to be aware of the scope their applications have from within the snap. Security policies and store policies work together to allow developers to quickly update their applications, and to provide safety to end users, and this document describes the sandbox and how to configure and work with th…

And the core security whitepaper:

https://developer.ubuntu.com/core/documentation