Start a migration guide

[sentrivana]

We can polish the wording/structure/split this away from the existing migration guide etc. later -- but we should already have a place to write down changes as we're making them.

[codecov]

Test Failures Detected: Due to failing tests, we cannot provide coverage reports at this time.

❌ Failed Test Results:

Completed 12221 tests with 1 failed, 10633 passed and 1587 skipped.

View the full list of failed tests

py3.7-common

• Class name: tests.profiler.test_continuous_profiler
  Test name: test_continuous_profiler_auto_start_and_manual_stop[experiment-thread]
  
  

  tests/profiler/test_continuous_profiler.py:204: in test_continuous_profiler_auto_start_and_manual_stop
      assert_single_transaction_with_profile_chunks(envelopes, thread)
  tests/profiler/test_continuous_profiler.py:110: in assert_single_transaction_with_profile_chunks
      assert len(items["profile_chunk"]) > 0
  E   assert 0 > 0
  E    +  where 0 = len([])