Extend schema to include alternative to a meetup group

[arkid15r]

Thank you @arkid15r !
Just a question, we wonder how do we specify an alternative meeting platform than Meetup?
As we state in our index, we use connpass for our chapter, as it is a popular platform among tech communities in Japan.

Thanks in advance.

Originally posted by @alterakey in OWASP/www-chapter-saitama#106 (comment)