Add the migration guide. (#672)

* Add migration guide to the readme.

Change-Id: I6834a1ac98f208ffad9fdedba392c25109a3d023

* add links

Change-Id: I992b94277878190aed5270adbb2b65d0f73423e6

* Fixing the colab button which was displayed on the same line as the next title

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Co-authored-by: Guillaume Vernade <[email protected]>

Author: MarkDaoust