Add a link to test the SSL certificate using SSL Labs

Until we fix issue #3 with real certificate checking, we could
provide a link to help IdPs diagnose potential issues with their
certificates.

The Qualys SSL Labs test tool is quite good:

  https://www.ssllabs.com/ssltest/index.html
Merge pull request #4 from fmarier/link_to_ssllabs

Add a link to test the SSL certificate using SSL Labs

Thanks @fmarier! r+