Add upgrade guide for pyresample 2.0

[djhoese]

User's should be given a guide on how to upgrade their code to use pyresample 2.0 when it is released. Things to include:

• Convert legacy area file format to YAML (@mraspaud built a conversion method iirc)
• Show old AreaDefinition to new AreaDefinition creation (Deprecate proj_id in AreaDefinition #153)

[michiboo]

Hi! Can I try this issue?

[djhoese]

@michiboo I am fine if you want to start it, but we are pretty far away from a real version 2.0 so I'm not sure how much you'd be able to write.