Add README.md for Shared/RepoTesting

tests/Shared/RepoTesting/README.md

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+# Running tests outside-of-repo
+
+This provides support for running tests outside of the repo, for example on a helix agent.
+- For this you need the source of the tests, and any dependencies.
+- Instead of direct `ProjectReferences` to the various Aspire hosting, and component projects use
+`@(ComponentReferenceForTests)`, and @(NonComponentReferenceForTests)`.
+- These are converted to `ProjectReference` when `$(TestsRunningOutsideOfRepo) != true`.
+- But converted to `PackageReference` when `$(TestsRunningOutsideOfRepo) == true`.
+
+- To allow building such test projects, the build is isolated and patched to build outside the
+repo by adding appropriate `Directory.Build.{props,targets}`, and `Directory.Packages.props`
+        - and using a custom `nuget.config` which resolves the Aspire packages from the locally built packages
+        - and a `Directory.Packages.Versions.props` is generated with PackageVersions taken from the repo
+- This also adds properties named in `@(PropertyForHelixRun)` from the repo, like `$(NetCurrent)`.
+
+## Adding new package versions
+
+Add any new package versions used by test projects in `tests/Shared/RepoTesting/Directory.Packages.Helix.props`.