Cross-link to Nothing GDK for Unity in this README.md

[am1goo]

I've decided to bring Nothing GDK into Unity Engine and as I can see it would be good possibility to each developer integrate specific
Glyph hardware-based features into their games.
So it will be great to add a chapter in README.md about linked platform/engine-based repos.
NothingOS.Unity is located here https://github.com/am1goo/NothingOS.Unity
I've already add link to this repo in my README.md file.

[JayKay135]

Yea that would be great. I've also already made a library for flutter: https://pub.dev/packages/nothing_glyph_interface

[am1goo]

@JayKay135 nice job, man!

[am1goo]

Changes in pull request #8