Add a migration guide #235

Job	Run time
Construct Examples matrix	5s
Benchmarks / Construct Benchmarks matrix	2s
Cxx interop / Construct Cxx interop matrix	2s
Soundness / Check generated code	1m 23s
Soundness / Check imports have access level	5s
Soundness / Swift license headers check	7s
Unit tests / Construct unit test matrix	4s
Soundness / API breakage check	1m 12s
Soundness / Broken symlinks check	3s
Soundness / Documentation check	1m 20s
Soundness / Format check	33s
Soundness / License headers check	2s
Soundness / Python lint check	6s
Soundness / Shell check	38s
Soundness / Unacceptable language check	3s
Soundness / YAML lint check	6s
Benchmarks / Benchmarks / Linux (6.0)	2m 54s
Benchmarks / Benchmarks / Linux (nightly-6.1)	3m 16s
Benchmarks / Benchmarks / Linux (nightly-main)	2m 56s
Cxx interop / Cxx interop / Linux (6.0)	59s
Cxx interop / Cxx interop / Linux (nightly-6.1)	1m 31s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 9s
Examples / Linux (6.0)	16m 45s
Examples / Linux (nightly-6.1)	4m 16s
Examples / Linux (nightly-main)	4m 32s
Unit tests / Unit tests / Linux (6.0)	1m 27s
Unit tests / Unit tests / Linux (nightly-6.1)	1m 47s
Unit tests / Unit tests / Linux (nightly-main)	1m 59s
	49m 22s

Provide feedback