Add section "Security Considerations"

[cportele]

It is good practice to provide security related guidance in a standard in a section "Security Considerations". Add such a section and include results from implementation experiences including from OGC Testbed 14.

(Raised by the OGC Architecture Board, OAB)

[cmheazel]

First draft has been added as Section 11

[cportele]

@cmheazel - instead of commiting this to the master branch, could you move this to a separate branch and create a pull request? Thanks!

[cmheazel]

Have backed out section 11 and created a Security-Updates branch. I will try to weave together all of the security related parts of the spec prior to issuing a pull request.