add a light README for testing

modules/find/tests/README.md

+Test MORSE CMake modules
+========================
+
+Procedure to test the `find_package' modules hosted in `modules/find`
+
+```
+git clone https://gitlab.inria.fr/solverstack/morse_cmake.git
+cd morse_cmake/modules/find/tests
+mkdir build
+cd build
+# to test all the packages, e.g.
+cmake .. -DENABLE_CTEST=ON
+# to test some only, e.g.
+cmake .. -DPACKAGES="HWLOC;STARPU;PASTIX"
+# to enable some components in some find, e.g. here we look for QUARK linked with HWLOC and PASTIX with PARSEC and STARPU
+cmake .. -DENABLE_CTEST=ON -DQUARK_COMPONENTS="HWLOC" -DPASTIX_COMPONENTS="PARSEC;STARPU"
+```