Upgrade `omniauth-saml` to 1.5.0 and document it's new capabilities.
With this MR I'm upgrading omniauth-saml
to 1.5.0 to include new and improved functionality. See https://github.com/omniauth/omniauth-saml/blob/master/CHANGELOG.md for more details.
This MR includes new documentation, so @axil can you also take a look?
/cc @DouweM