Add package README for MSTest.SourceGeneration (#9345)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Evangelink

src/Analyzers/MSTest.SourceGeneration/PACKAGE.md

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+# MSTest.SourceGeneration
+
+MSTest is Microsoft supported Test Framework.
+
+This package provides the C# source generators that emit reflection metadata so that MSTest test assemblies can run trim-safe and NativeAOT-compatible.
+
+For access to the testing framework, install the MSTest.TestFramework package.