Add example on how to combat clickjacking

Give example on how to make Apache provide clickjacking
protection, by sending the `X-Frame-Options` header.

Ref h5bp/server-configs-apache#8.
Close h5bp/server-configs-apache#9.
Send `X-Frame-Options` header only for web pages

Sending the `X-Frame-Options` header for everything by default,
doesn't bring any value when it comes to providing clickjacking
protection.

Even if some developers may want to send this header for some
other content (e.g.: images) to block other websites from using
it, for the majority that won't be the case.

Ref: h5bp/server-configs-apache#8
     h5bp/server-configs-apache#9
Improve inline comments about `X-Frame-Options`

Provide more information on the advantages and disadvantages of sending
the `X-Frame-Options` header, but also inform on what kind of web pages
it shoud be send for.

Ref: h5bp/server-configs-apache#8
     h5bp/server-configs-apache#9