Update with Discovery and Directory recommendations

[mmccool]

Once discovery is drafted, the security documents need to be updated to give best practices and security and privacy considerations for them.

[mmccool]

We should talk about:

• When "no security" is appropriate (if it is at all)
• Appropriate security for different contexts (home hub, smart city services, industrial, etc).

[mmccool]

We also need to update the current "Thing Directory" section to use the correct/current terminology (Thing Description Directory).