Add Documentation for "Strict-Transport-Security" header in Knox responses

The HTTP Strict-Transport-Security response header is a security feature that lets a web site tell browsers that it should only be communicated with using HTTPS, instead of using HTTP. Possible values are:

• max-age=<expire-time>
• max-age=<expire-time>; includeSubDomains
• max-age=<expire-time>; preload