Add a migration guide from OmniAuth 0.3

Add undocumented configuration option

I had a hard time figuring out how to configure the correct endpoints. Maybe it helps someone.

disable_ssl_verification is currently undocumented.

https://github.com/jeromegn/DocumentUp/issues/23#issuecomment-4821497

Configuring ssl certs path for ubuntu

Thanks to @iterateNZ and @jbbarth for reporting and helping to
troubleshoot the issue.