add advice following discussions with Liz Hare

[hturner]

This builds on previous recommendations we had for making events accessible for people with visual impairments. Following discussions with Liz Hare, I've added

• further tips for conference organizers
• advice for presenters (split out into speakers, tutors and flash poster presenters)
• feedback on the website/app used to communicate the useR! 2019 schedule

There is much that still needs to be completed following further discussion and testing, I thought that making a pull request might make it easier for people to comment on specific parts and suggest concrete changes.

[hturner]

Things to add (feedback from useR! surveys):

• quiet space (that people can find!)
• seating in break areas
• tables for eating food

[hturner]

Another point from Liz: if mobile app and/or website schedule not accessible, at least provide plain text file or other format (.csv?) that would be easier to navigate.