Add ExternalSampleProjects minimal README

Author: jfreire-unity

ExternalSampleProjects/README.md

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+# External Sample Projects
+
+These Samples are self-contained projects because they are too big to be included as a package Sample.
+
+When editing these projects, you need to manually add a few packages for them to work.
+The projects don't contain the packages themselves.
+
+The `InputDeviceTester` needs:
+
+- Input System package (latest version)
+- Unity UI package (2.0.0 version at least)
+- Jetbrains Rider Editor / Visual Studio package for IDE tooling.
+
+The `TouchSamples` need:
+
+- All the same as ´InputDeviceTester`
+- Cinemachine (2.10.3 maximum; 3.x has errors with the project but are easy to fix)
+- Probuilder (5.2.3 version maximum at the moment; 6.x has problems building for iOS and Android)