proposals: add diversity statement

Adapted from previous own words from http://2021.worldhaptics.org/sic/

We encourage participation from students from communities under-represented in the Science, Technology, Engineering, Arts, and Mathematics (STEAM) fields. We aim at increasing the participation of groups traditionally underrepresented in software development (including but not limited to: women, LGBTQ+, underrepresented minorities, and people with disabilities).