Update `README.md`

Include a short description of the project, as well as information on how to use
the configurations and what Apache and browser versions are supported.
Use DENY as the default value for X-Frame-Options

The role of the security related examples is to provide the most secure
way of doing things.

In the case of `X-Frame-Options`, it turns out that invisible site-wide
XSS attacks might bypass the header if it's send with the more tolerant
value of `SAMEORIGIN`.

Ref: https://cure53.de/xfo-clickjacking.pdf