Google seems to have presumably started the rollout of Android 6.0.1 Marshmallow for its Android One range of smartphones. As you may know, Android One handsets are mainly sold in developing markets (by local companies).
Early reports indicate that Android 6.0.1 does not really bring anything new to the table. It is likely that the update only contains some bug fixes and stability improvements.
HTC ROM developer @LlabTooFeR has it that, after 6.0.1, Google is planning to roll out Android 6.1 Marshmallow. Needless to say, Android 6.1 should be a more important update compared to 6.0.1.
