Add security section - WIP.
Added sections 'Involving the Security Team' and 'X-Frame-Options'
fixed minor issues with the CSP section and rewrote the bullet points to a field list
Finished CSP section and added projects simplifying the use of CSP.
Using local copy of security.
Restore freddyb's changes.
Fix comma, adjust wording.
Major text-shortening to make it more relevant to Mozillians.
Use consistent two newline style
Use "element" not "elmt"
Don't say not to use CSP
Simplify wording
Merge branch 'qll/csp-and-security'