write a 0.2.x -> 0.3.0 migration guide

[japaric]

0.3 will include several breaking changes. this guide would explain how to migrate code that uses defmt 0.2.x to 0.3.0

example of a migration guide: https://rtic.rs/0.5/book/en/migration.html

[Urhengulas]

Should this guide only list breaking changes, or also new features?

[japaric]

the focus should be on fixing things that no longer compile or no longer behave the same: changes in syntax; changes in API; defmt-* Cargo features no longer being needed; DEFMT_LOG now being a thing. It should offer a quick fix (e.g. to get the old behavior use DEFMT_LOG=info cargo r), ideally in the form of a diff when code is involved. To avoid repetition, the migration guide should defer / link to other dedicated sections of the book for details about new features (e.g. ISO timestamp display hint).