Update Readme to point to http://aka.ms/ros/mrtk for Hololens support

[hoffmann-stefan]

@ooeygui pointed out that the current codebase for using ROS 2 on Hololens is over at http://aka.ms/ros/mrtk_native and http://aka.ms/ros/mrtk, see comment #109 (comment)

After looking at it seems the tooling and install instructions over there are way better and more up to date.
Instead of having some duplicate install and setup instructions for UWP/Hololens in the README, we should simply point to the ros_msft_mrtk repo.

[esteve]

@hoffmann-stefan I'd prefer if the references to ros2-dotnet supporting UWP were not removed, I worked quite hard to make sure that all the components in ROS 2 supported UWP (i.e. FastRTPS/FastDDS, rmw, rcl, ros2-dotnet, etc.)

@ooeygui could you clarify what was missing in this repository to support UWP? I asked you in #82 (comment) because five years ago we already demoed ros2-dotnet running onboard the Hololens 1, and did all the work to port ROS 2 and ros2-dotnet to UWP

[esteve]

@hoffmann-stefan for reference, this is the talk I gave at ROSCon in 2018 about adding support for UWP to ROS 2 ROS2 for Android, iOS and Universal Windows Platform

[hoffmann-stefan]

@esteve I added a section to the README.md that should better explain the UWP support and how this library contributes to this. The point of this PR is to point to the tooling and install instructions of the Mixed Reality Toolkit for ROS 2, not to remove the references to the UWP support.

Does this address your concerns?