Fix README
Use a custom Diff type

This has no perceptible effect on performance, but using a plain
old tuple just kind of offends my tender sensibilities :-(

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extra : amend_source : 2f59337a60b9dcbac9c912b077c6660152a9a11f
Bump version to 0.2.0.0

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extra : histedit_source : 3fd3bcc5e0df940168c528d5d47e297ed8a24aaf
Added tag 0.2.0.0 for changeset 4fd4c4dab39c