DOC: Add the DIPY banner to the README file.
DOC: Add the DIPY banner to the README file.

Add the **DIPY** banner to the `README.rst` file to strenghten the
visual appeal of the repository and help the community better identify
the toolkit.

nilgoyyou/dipy,nilgoyyou/dipy,FrancoisRheaultUS/dipy,FrancoisRheaultUS/dipy