chore(docs): update migration README.md (#1003)

# Description

Thank you for opening a Pull Request!
Before submitting your PR, there are a few things you can do to make
sure it goes smoothly:

- [x] Follow the [Contribution
Guide](https://github.com/a2aproject/a2a-python/blob/main/CONTRIBUTING.md).
- [x] Make your Pull Request title in the
<https://www.conventionalcommits.org/> specification.
- Important Prefixes for
[release-please](https://github.com/googleapis/release-please):
- `fix:` which represents bug fixes, and correlates to a
[SemVer](https://semver.org/) patch.
- `feat:` represents a new feature, and correlates to a SemVer minor.
- `feat!:`, or `fix!:`, `refactor!:`, etc., which represent a breaking
change (indicated by the `!`) and will result in a SemVer major.
- [x] Ensure the tests and linter pass (Run `bash scripts/format.sh`
from the repository root to format)
- [x] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

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Co-authored-by: Iva Sokolaj <102302011+sokoliva@users.noreply.github.com>
Co-authored-by: sokoliva <sokolaj@google.com>

Author: msampathkumar