[Documentation] Add security guide

[VWoeltjen]

Add a security guide to address #1833. Handled this as a separate document, rephrasing the concerns identified in that document for an audience of plugin developers and system administrators.

[psarram]

This is a very good guide. One question, is it worth mentioning that we also run JSLint which checks for some security vulnerabilities?

[VWoeltjen]

Author Checklist

1. Changes address original issue? Y
2. Unit tests included and/or updated with changes? N/A (documentation only)
3. Command line build passes? Y
4. Changes have been smoke-tested? Y

[psarram]

Reviewer Checklist

1. Changes appear to address issue? Y
2. Appropriate unit tests included? N/A
3. Code style and in-line documentation are appropriate? N/A
4. Commit messages meet standards? Y