Added StringPool info in package description

Author: Sergio0694

Microsoft.Toolkit.HighPerformance/Microsoft.Toolkit.HighPerformance.csproj

@@ -12,6 +12,7 @@
       - MemoryBufferWriter<T>: an IBufferWriter<T>: implementation that can wrap external Memory<T>: instances.
       - MemoryOwner<T>: an IMemoryOwner<T> implementation with an embedded length and a fast Span<T> accessor.
       - SpanOwner<T>: a stack-only type with the ability to rent a buffer of a specified length and getting a Span<T> from it.
+      - StringPool: a configurable pool for string instances that be used to minimize allocations when creating multiple strings from char buffers.
       - String, array, Span<T>, Memory<T> extensions and more, all focused on high performance.
       - HashCode<T>: a SIMD-enabled extension of HashCode to quickly process sequences of values.
       - BitHelper: a class with helper methods to perform bit operations on numeric types.