Add info on dealing with speaker cancellations

[tomschenkenberg]

Inspired by this post:
https://mevanbabakar.com/2019/10/16/conference-diversity-your-responsibility/

"If you actually believe that diversity is important at your conference, you won’t pin your hopes of diversity on one speaker."

and

"Don’t make your one BME or female speaker feel like they are shouldering the burden of diversity for the entire event. Diversity is your responsibility."