[Admin] Establish sizing for tasks

[HuangJunye]

Adding size to all issues and added size info on project board readme.

Sizing

We each work approximately 5h/week = 1h/day

• XS: 1 day = 1h
• S: 1\~3 days = 1\~3h
• M: 1 week = 5h
• L: 2 weeks = 10h
• XL: 4 weeks = 20h

Each sprint = 1L = 2M = 4S = 8XS