Add tools and refs.

[righettod]

Hi,

This PR perform the following updates:

• Add a reference to the documentation page of the Next.js framework about how to specify http response headers.
• Add a tools to check for HTTP response security headers.
  • I have found it by searching for tools and libs on Google in order to promote free and open sources projects focusing on HTTP security headers. I tested it and it seems very interesting 😉

Thanks in advance 😀