Add readme for exercises

Author: Mathieu Lobet

exercises/README.md

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+# Exercises
+
+- [Exercise 1: Compiling Kokkos](01_compiling_kokkos/README.md)
+- [Exercise 2: First Kokkos Program](02_first_program/README.md)
+- [Exercise 3: Learn how to use and manage basic Kokkos View](03_basic_view/README.md)
+- [Exercise 4: Mirror view and deep copy](04_deep_copy/README.md)
+- [Exercise 5: Parallel Loop](05_parallel_loop/README.md)
+- [Exercise 6: Parallel Reduce](06_parallel_reduce/README.md)