Add a small README inside the InternalImmortals dir

InternalImmortals/README.md

@@ -0,0 +1,12 @@
+
+Internal Samples for going deeper with AMBROSIA
+===============================================
+
+This directory contains internal examples and performance tests that
+must be built inside a full check-out of AMBROSIA.
+
+While the `Samples/` directory contains representative samples for a
+user building a new application, this directory is more for AMBROSIA
+developers, or those interested in work-in-progress forthcoming
+features.
+