Docs: Solution for using multiple private repos

[dumityty]

While trying to figure out how to use the action on a composer file with multiple private repos, I luckily stumbled upon the issue https://github.com/php-actions/composer/issues/10 where it explained using a normal user instead of deploy keys. That was going to be my next step in testing out a solution and was good to see it confirmed by somebody else who needed this. There was a mention of updating the doc to mention this, so i thought i'd update it and send a PR over. Let me know if something doesn't make sense or would like to say something else in it.