Mention Frozen.* types in PACKAGE.md for System.Collections.Immutable (…

…dotnet#93541)

src/libraries/System.Collections.Immutable/src/PACKAGE.md

@@ -53,6 +53,10 @@ The main types provided by this library are:
 * `System.Collections.Immutable.ImmutableSortedSet<T>`
 * `System.Collections.Immutable.ImmutableStack`
 * `System.Collections.Immutable.ImmutableStack<T>`
+* `System.Collections.Frozen.FrozenDictionary`
+* `System.Collections.Frozen.FrozenDictionary<TKey, TValue>`
+* `System.Collections.Frozen.FrozenSet`
+* `System.Collections.Frozen.FrozenSet<T>`
 
 ## Additional Documentation
 
@@ -65,4 +69,4 @@ The main types provided by this library are:
 
 <!-- How to provide feedback on this package and contribute to it -->
 
-System.Collections.Immutable is released as open source under the [MIT license](https://licenses.nuget.org/MIT). Bug reports and contributions are welcome at [the GitHub repository](https://github.com/dotnet/runtime).
+System.Collections.Immutable is released as open source under the [MIT license](https://licenses.nuget.org/MIT). Bug reports and contributions are welcome at [the GitHub repository](https://github.com/dotnet/runtime).