Android 6.0 Marshmallow was launched 2 about a month and a half back and Google has already started to seed the Android 6.0.1 Marshmallow to some Android One devices.
The update includes the December security update and it will bring some performance improvements and bug fixes. The Cherry Mobile G1 in Philipines is the first mobile to receive this update. Other Android devices around the globe should get the update soon. And not to forget the Nexus devices, they will also join the update stream very soon. Other than Nexus devices owners might have to wait for few months to even get this minor update. It is possible that Google will launch another update till then and the manufacturers will release that update instead of 6.0.1. Like most of the manufacturers updated their devices to Android 5.1 directly, skipping Android 5.0.
Android One is the project from Google that was introduced last year. Android devices are just like the Nexus devices but with lower specifications. Android One devices cost around $100 and they receive the quick updates just like Nexus devices for 2 years.