Add link do Composer documentation from Security tab

[Marc Würth]

I think it would help Composer newbies, if you would add a link to the Composer documentation in the following message:

"For a composer.json file we don't know exactly which versions are locked on production. We assume that all version constraints are resolved to the newest version. For a more accurate resolution we need the composer.lock file. "

There are two relevant links:

• https://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file
• https://getcomposer.org/doc/02-libraries.md#lock-file (which is linked at the end of the first)

[Robert Reiz]

Makes totally sense. I just added it. It goes online with the next deployment today.