Update CHANGELOG.md

fixed LunaGao#41

support nullsafety

upgrade to 3.2.0

Merge pull request LunaGao#36 from LunaGao/feature/LunaGao#35

fixed LunaGao#35

update flutter to 1.20.2

fixed some web platform bugs

Merge pull request LunaGao#29 from LunaGao/develop

Develop

Merge pull request LunaGao#14 from LunaGao/develop

using `Flag` widget to instead of `Flags.getFlag`

Merge pull request LunaGao#13 from LunaGao/develop

fixed LunaGao#12