Recognizing there are multiple ways in which developers want to be able to get access to project through Unity, the Mixed Reality Toolkit has been engineered to be available through the following methods:
You can download the packaged Unity Asset from here for importing in to your project.
Unity now has an asset plugin for getting access to Unity projects using the dependency delivery solution known as NuGet.
Simply install the Unity NuGet package and search for the Mixed Reality Toolkit to download the project.
As soon as it's available, the MRTK team will be working with Unity to publish the Mixed Reality Toolkit as a package through their Unity Package Manager.
It has been raised within the Mixed Reality Toolkit team that the package should be published to the Unity store, this is currently being investigated.
Feedback is welcome through the Mixed Reality GitHub site (raise an issue to offer an opinion) and the HoloDevelopers Slack channel for the Mixed Reality Toolkit.
For those who would like to contribute to the MRTK or prefer to have the latest code in their project, there is another way to get access to the latest and greatest of the Mixed Reality Toolkit, be it the Release code or the in-progress development of the project.
Stephen Hodgson has provided a full guide for how to use Git Submodules to download and synchronize the toolkit in to your project. Don't worry though, you still have control on how you update your project and can even "Go back" to a previous version of the toolkit at any time by simply checking out a specific commit or tag.
If you feel there are other options not listed above that would be beneficial to Unity developers, please let us know on the links above