Write a page on the security in Drive client

The new documentation page will address the following questions:

1. Functional description of the agent, and its interfaces. Who does it need to talk to, and what are the minimal permissions it needs on the hosting system?

2. How can we ensure secure by default installation (i.e. filesystem ACLs, not running unnecessary services, running with minimal privileges, not opening unneeded listening ports etc, data only flows to intended endpoints, and doesn't sync to any unexpected locations)?

3. What is the recommended patching strategy for updates and security fixes?