Add ex. on how to reduce MIME type security risks

For compatibility reasons (e.g.: legacy servers that serve all files
as `text/plain`), IE 8+ has a MIME-sniffing feature that will attempt
to determine the content-type for each downloaded resource.

In some cases, IE may report a different MIME type than the type speci-
fied by the web server. For instance, if IE finds HTML content in a
file delivered with the HTTP response header `Content-Type: text/plain`,
it determines that the content should be rendered as HTML. This can lead
to security problems especially for servers hosting untrusted content.

Fortunately, IE provides web apps with the ability to opt-out of
MIME-sniffing by sending the `X-Content-Type-Options` response header
with the value `nosniff`.

Beside IE 8+, this feature has recently been implemented in Chrome, and
may soon come to Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=471020.

See also:
  * http://msdn.microsoft.com/en-us/library/ie/gg622941
  * http://blogs.msdn.com/b/ie/archive/2008/09/02/ie8-security-part-vi-beta-2-update.aspx

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