DOC: Add a note in API changes about dropping 2.6 support.
DOC: Add a note in API changes about dropping 2.6 support.

nilgoyyou/dipy,FrancoisRheaultUS/dipy,FrancoisRheaultUS/dipy,villalonreina/dipy,nilgoyyou/dipy,matthieudumont/dipy,matthieudumont/dipy,villalonreina/dipy