added a section on secure access to the CDN to the docs
added a section on secure access to the CDN to the docs

mitya57/mathjax-docs,mathjax/MathJax-docs,GerHobbelt/MathJax-docs,GerHobbelt/MathJax-docs,Axure/MathJax-docs,mathjax/MathJax-docs,Axure/MathJax-docs,mitya57/mathjax-docs