add Q# package registry document

library/Registry.md

@@ -0,0 +1,22 @@
+# Q# Package Registry
+
+This document is a list of notable Q# library projects. If you have a package that you would like to add to this list, please open a pull request.
+
+## Unstable
+
+[link](https://github.com/microsoft/qsharp/tree/main/library/unstable)
+
+A general staging ground for some useful APIs that have not yet been stabailized into the standard library.
+
+## Signed
+
+[link](https://github.com/microsoft/qsharp/tree/main/library/signed)
+
+Defines types and functions to work with signed qubit-based integers.
+
+## FixedPoint
+
+[link](https://github.com/microsoft/qsharp/tree/main/library/fixed_point)
+
+Types and functions for fixed-point arithmetic with qubits.
+