Added: LDM Notes for June 21st, 2021

[333fred]

https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-06-21.md

Agenda

1. Open questions for lambda return types
2. List patterns in recursive patterns
3. Open questions in async method builder
4. Email Decision: Duplicate global using warnings

[333fred]

I've clarified that the notes on lambda return type variance apply to all scenarios, not just method type inference, and added notes for an email decision on duplicate global using warnings.